[xiph-commits] r18828 - websites/xiph.org/video
xiphmont at svn.xiph.org
xiphmont at svn.xiph.org
Mon Feb 25 15:17:42 PST 2013
Author: xiphmont
Date: 2013-02-25 15:17:42 -0800 (Mon, 25 Feb 2013)
New Revision: 18828
Added:
websites/xiph.org/video/vid2-en.vtt
websites/xiph.org/video/vid2-poster-360.jpg
websites/xiph.org/video/vid2-poster-480.jpg
websites/xiph.org/video/vid2-poster-720.jpg
websites/xiph.org/video/vid2.shtml
Log:
Add live vid2 test page, not yet linked
Added: websites/xiph.org/video/vid2-en.vtt
===================================================================
--- websites/xiph.org/video/vid2-en.vtt (rev 0)
+++ websites/xiph.org/video/vid2-en.vtt 2013-02-25 23:17:42 UTC (rev 18828)
@@ -0,0 +1,1773 @@
+WEBVTT
+
+1
+00:00:08.252 --> 00:00:11.550
+Hi, I'm Monty Montgomery from Red Hat and Xiph.Org.
+
+2
+00:00:11.550 --> 00:00:18.430
+A few months ago, I wrote an article on digital audio and why 24bit/192kHz music downloads don't make sense.
+
+3
+00:00:18.430 --> 00:00:23.433
+In the article, I mentioned--almost in passing-- that a digital waveform is not a stairstep,
+
+4
+00:00:23.433 --> 00:00:28.680
+and you certainly don't get a stairstep when you convert from digital back to analog.
+
+5
+00:00:29.865 --> 00:00:33.865
+Of everything in the entire article, *that* was the number one thing people wrote about.
+
+6
+00:00:33.865 --> 00:00:37.221
+In fact, more than half the mail I got was questions and comments
+
+7
+00:00:37.221 --> 00:00:39.663
+about basic digital signal behavior.
+
+8
+00:00:39.894 --> 00:00:45.285
+Since there's interest, let's take a little time to play with some _simple_ digital signals.
+
+9
+00:00:49.747 --> 00:00:51.006
+Pretend for a moment
+
+10
+00:00:51.006 --> 00:00:54.089
+that we have no idea how digital signals really behave.
+
+11
+00:00:54.734 --> 00:00:56.841
+In that case it doesn't make sense for us
+
+12
+00:00:56.841 --> 00:00:59.049
+to use digital test equipment either.
+
+13
+00:00:59.049 --> 00:01:00.937
+Fortunately for this exercise, there's still
+
+14
+00:01:00.937 --> 00:01:04.020
+plenty of working analog lab equipment out there.
+
+15
+00:01:04.020 --> 00:01:05.972
+First up, we need a signal generator
+
+16
+00:01:05.972 --> 00:01:08.190
+to provide us with analog input signals--
+
+17
+00:01:08.190 --> 00:01:12.692
+in this case, an HP3325 from 1978.
+
+18
+00:01:12.692 --> 00:01:14.153
+It's still a pretty good generator,
+
+19
+00:01:14.153 --> 00:01:15.614
+so if you don't mind the size,
+
+20
+00:01:15.614 --> 00:01:16.532
+the weight,
+
+21
+00:01:16.532 --> 00:01:17.577
+the power consumption,
+
+22
+00:01:17.577 --> 00:01:18.910
+and the noisy fan,
+
+23
+00:01:18.910 --> 00:01:20.329
+you can find them on eBay.
+
+24
+00:01:20.329 --> 00:01:23.863
+Occasionally for only slightly more than you'll pay for shipping.
+
+25
+00:01:24.617 --> 00:01:28.500
+Next, we'll observe our analog waveforms on analog oscilloscopes,
+
+26
+00:01:28.500 --> 00:01:31.550
+like this Tektronix 2246 from the mid-90s,
+
+27
+00:01:31.550 --> 00:01:34.761
+one of the last and very best analog scopes ever made.
+
+28
+00:01:34.761 --> 00:01:36.807
+Every home lab should have one.
+
+29
+00:01:37.716 --> 00:01:40.852
+And finally inspect the frequency spectrum of our signals
+
+30
+00:01:40.852 --> 00:01:43.177
+using an analog spectrum analyzer.
+
+31
+00:01:43.177 --> 00:01:47.732
+This HP3585 from the same product line as the signal generator.
+
+32
+00:01:47.732 --> 00:01:50.615
+Like the other equipment here it has a rudimentary
+
+33
+00:01:50.615 --> 00:01:52.905
+and hilariously large microcontroller,
+
+34
+00:01:52.905 --> 00:01:56.276
+but the signal path from input to what you see on the screen
+
+35
+00:01:56.276 --> 00:01:58.537
+is completely analog.
+
+36
+00:01:58.537 --> 00:02:00.329
+All of this equipment is vintage,
+
+37
+00:02:00.329 --> 00:02:01.993
+but aside from its raw tonnage,
+
+38
+00:02:01.993 --> 00:02:03.844
+the specs are still quite good.
+
+39
+00:02:04.536 --> 00:02:06.868
+At the moment, we have our signal generator
+
+40
+00:02:06.868 --> 00:02:12.829
+set to output a nice 1kHz sine wave at one volt RMS,
+
+41
+00:02:13.414 --> 00:02:15.220
+we see the sine wave on the oscilloscope,
+
+42
+00:02:15.220 --> 00:02:21.428
+can verify that it is indeed 1kHz at one volt RMS,
+
+43
+00:02:21.428 --> 00:02:24.108
+which is 2.8V peak-to-peak,
+
+44
+00:02:24.308 --> 00:02:27.561
+and that matches the measurement on the spectrum analyzer as well.
+
+45
+00:02:27.561 --> 00:02:30.644
+The analyzer also shows some low-level white noise
+
+46
+00:02:30.644 --> 00:02:32.190
+and just a bit of harmonic distortion,
+
+47
+00:02:32.190 --> 00:02:36.649
+with the highest peak about 70dB or so below the fundamental.
+
+48
+00:02:36.649 --> 00:02:38.612
+Now, this doesn't matter at all in our demos,
+
+49
+00:02:38.612 --> 00:02:40.574
+but I wanted to point it out now
+
+50
+00:02:40.574 --> 00:02:42.452
+just in case you didn't notice it until later.
+
+51
+00:02:44.036 --> 00:02:47.142
+Now, we drop digital sampling in the middle.
+
+52
+00:02:48.557 --> 00:02:51.024
+For the conversion, we'll use a boring,
+
+53
+00:02:51.024 --> 00:02:53.374
+consumer-grade, eMagic USB1 audio device.
+
+54
+00:02:53.374 --> 00:02:55.337
+It's also more than ten years old at this point,
+
+55
+00:02:55.337 --> 00:02:57.257
+and it's getting obsolete.
+
+56
+00:02:57.964 --> 00:03:02.676
+A recent converter can easily have an order of magnitude better specs.
+
+57
+00:03:03.076 --> 00:03:07.924
+Flatness, linearity, jitter, noise behavior, everything...
+
+58
+00:03:07.924 --> 00:03:09.353
+you may not have noticed.
+
+59
+00:03:09.353 --> 00:03:11.604
+Just because we can measure an improvement
+
+60
+00:03:11.604 --> 00:03:13.609
+doesn't mean we can hear it,
+
+61
+00:03:13.609 --> 00:03:16.404
+and even these old consumer boxes were already
+
+62
+00:03:16.404 --> 00:03:18.643
+at the edge of ideal transparency.
+
+63
+00:03:20.244 --> 00:03:22.825
+The eMagic connects to my ThinkPad,
+
+64
+00:03:22.825 --> 00:03:26.121
+which displays a digital waveform and spectrum for comparison,
+
+65
+00:03:26.121 --> 00:03:28.788
+then the ThinkPad sends the digital signal right back out
+
+66
+00:03:28.788 --> 00:03:30.921
+to the eMagic for re-conversion to analog
+
+67
+00:03:30.921 --> 00:03:33.332
+and observation on the output scopes.
+
+68
+00:03:33.332 --> 00:03:35.582
+Input to output, left to right.
+
+69
+00:03:40.211 --> 00:03:41.214
+OK, it's go time.
+
+70
+00:03:41.214 --> 00:03:43.924
+We begin by converting an analog signal to digital
+
+71
+00:03:43.924 --> 00:03:47.347
+and then right back to analog again with no other steps.
+
+72
+00:03:47.347 --> 00:03:49.268
+The signal generator is set to produce
+
+73
+00:03:49.268 --> 00:03:52.649
+a 1kHz sine wave just like before.
+
+74
+00:03:52.649 --> 00:03:57.428
+We can see our analog sine wave on our input-side oscilloscope.
+
+75
+00:03:57.428 --> 00:04:01.694
+We digitize our signal to 16 bit PCM at 44.1kHz,
+
+76
+00:04:01.694 --> 00:04:03.828
+same as on a CD.
+
+77
+00:04:03.828 --> 00:04:07.156
+The spectrum of the digitized signal matches what we saw earlier. and...
+
+78
+00:04:07.156 --> 00:04:10.836
+what we see now on the analog spectrum analyzer,
+
+79
+00:04:10.836 --> 00:04:15.154
+aside from its high-impedance input being just a smidge noisier.
+
+80
+00:04:15.154 --> 00:04:15.956
+For now
+
+81
+00:04:18.248 --> 00:04:20.798
+the waveform display shows our digitized sine wave
+
+82
+00:04:20.798 --> 00:04:23.966
+as a stairstep pattern, one step for each sample.
+
+83
+00:04:23.966 --> 00:04:26.388
+And when we look at the output signal
+
+84
+00:04:26.388 --> 00:04:29.054
+that's been converted from digital back to analog, we see...
+
+85
+00:04:29.054 --> 00:04:32.052
+It's exactly like the original sine wave.
+
+86
+00:04:32.052 --> 00:04:33.483
+No stairsteps.
+
+87
+00:04:33.914 --> 00:04:37.193
+OK, 1kHz is still a fairly low frequency,
+
+88
+00:04:37.193 --> 00:04:40.633
+maybe the stairsteps are just
+hard to see or they're being smoothed away.
+
+89
+00:04:40.739 --> 00:04:49.492
+Fair enough. Let's choose
+a higher frequency, something close to Nyquist, say 15kHz.
+
+90
+00:04:49.492 --> 00:04:53.545
+Now the sine wave is represented by less than three samples per cycle, and...
+
+91
+00:04:53.545 --> 00:04:55.838
+the digital waveform looks pretty awful.
+
+92
+00:04:55.838 --> 00:04:59.798
+Well, looks can be deceiving. The analog output...
+
+93
+00:05:01.876 --> 00:05:06.033
+is still a perfect sine wave, exactly like the original.
+
+94
+00:05:06.633 --> 00:05:09.228
+Let's keep going up.
+
+95
+00:05:17.353 --> 00:05:20.151
+16kHz....
+
+96
+00:05:23.198 --> 00:05:25.616
+17kHz...
+
+97
+00:05:28.201 --> 00:05:29.945
+18kHz...
+
+98
+00:05:33.822 --> 00:05:35.548
+19kHz...
+
+99
+00:05:40.457 --> 00:05:42.465
+20kHz.
+
+100
+00:05:49.097 --> 00:05:52.350
+Welcome to the upper limits of human hearing.
+
+101
+00:05:52.350 --> 00:05:54.377
+The output waveform is still perfect.
+
+102
+00:05:54.377 --> 00:05:58.025
+No jagged edges, no dropoff, no stairsteps.
+
+103
+00:05:58.025 --> 00:06:01.342
+So where'd the stairsteps go?
+
+104
+00:06:01.342 --> 00:06:03.198
+Don't answer, it's a trick question.
+
+105
+00:06:03.198 --> 00:06:04.318
+They were never there.
+
+106
+00:06:04.318 --> 00:06:06.652
+Drawing a digital waveform as a stairstep
+
+107
+00:06:08.712 --> 00:06:10.772
+was wrong to begin with.
+
+108
+00:06:10.942 --> 00:06:11.998
+Why?
+
+109
+00:06:11.998 --> 00:06:14.366
+A stairstep is a continuous-time function.
+
+110
+00:06:14.366 --> 00:06:16.201
+It's jagged, and it's piecewise,
+
+111
+00:06:16.201 --> 00:06:19.700
+but it has a defined value at every point in time.
+
+112
+00:06:19.700 --> 00:06:22.004
+A sampled signal is entirely different.
+
+113
+00:06:22.004 --> 00:06:23.337
+It's discrete-time;
+
+114
+00:06:23.337 --> 00:06:27.337
+it's only got a value right at each instantaneous sample point
+
+115
+00:06:27.337 --> 00:06:32.596
+and it's undefined, there is no value at all, everywhere between.
+
+116
+00:06:32.596 --> 00:06:36.666
+A discrete-time signal is properly drawn as a lollipop graph.
+
+117
+00:06:40.020 --> 00:06:42.974
+The continuous, analog counterpart of a digital signal
+
+118
+00:06:42.974 --> 00:06:45.364
+passes smoothly through each sample point,
+
+119
+00:06:45.364 --> 00:06:50.153
+and that's just as true for high frequencies as it is for low.
+
+120
+00:06:50.153 --> 00:06:53.033
+Now, the interesting and not at all obvious bit is:
+
+121
+00:06:53.033 --> 00:06:55.454
+there's only one bandlimited signal that passes
+
+122
+00:06:55.454 --> 00:06:57.417
+exactly through each sample point.
+
+123
+00:06:57.417 --> 00:06:58.708
+It's a unique solution.
+
+124
+00:06:58.708 --> 00:07:01.246
+So if you sample a bandlimited signal
+
+125
+00:07:01.246 --> 00:07:02.612
+and then convert it back,
+
+126
+00:07:02.612 --> 00:07:06.462
+the original input is also the only possible output.
+
+127
+00:07:06.462 --> 00:07:07.838
+And before you say,
+
+128
+00:07:07.838 --> 00:07:11.721
+"Oh, I can draw a different signal that passes through those points."
+
+129
+00:07:11.721 --> 00:07:14.283
+Well, yes you can, but...
+
+130
+00:07:17.268 --> 00:07:20.521
+if it differs even minutely from the original,
+
+131
+00:07:20.521 --> 00:07:24.905
+it contains frequency content at or beyond Nyquist,
+
+132
+00:07:24.905 --> 00:07:26.185
+breaks the bandlimiting requirement
+
+133
+00:07:26.185 --> 00:07:28.358
+and isn't a valid solution.
+
+134
+00:07:28.574 --> 00:07:30.036
+So how did everyone get confused
+
+135
+00:07:30.036 --> 00:07:32.702
+and start thinking of digital signals as stairsteps?
+
+136
+00:07:32.702 --> 00:07:34.900
+I can think of two good reasons.
+
+137
+00:07:34.900 --> 00:07:37.956
+First: It's easy enough to convert a sampled signal
+
+138
+00:07:37.972 --> 00:07:39.294
+to a true stairstep.
+
+139
+00:07:39.294 --> 00:07:42.409
+Just extend each sample value forward until the next sample period.
+
+140
+00:07:42.409 --> 00:07:44.414
+This is called a zero-order hold,
+
+141
+00:07:44.414 --> 00:07:47.913
+and it's an important part of how some digital-to-analog converters work,
+
+142
+00:07:47.913 --> 00:07:50.089
+especially the simplest ones.
+
+143
+00:07:50.089 --> 00:07:55.591
+So, anyone who looks up digital-to-analog conversion
+
+144
+00:07:55.592 --> 00:07:59.550
+is probably going to see a diagram of a stairstep waveform somewhere,
+
+145
+00:07:59.550 --> 00:08:01.982
+but that's not a finished conversion,
+
+146
+00:08:01.982 --> 00:08:04.250
+and it's not the signal that comes out.
+
+147
+00:08:04.944 --> 00:08:05.684
+Second,
+
+148
+00:08:05.684 --> 00:08:07.529
+and this is probably the more likely reason,
+
+149
+00:08:07.529 --> 00:08:09.449
+engineers who supposedly know better,
+
+150
+00:08:09.449 --> 00:08:10.441
+like me,
+
+151
+00:08:10.441 --> 00:08:13.193
+draw stairsteps even though they're technically wrong.
+
+152
+00:08:13.193 --> 00:08:15.571
+It's a sort of like a one-dimensional version of
+
+153
+00:08:15.571 --> 00:08:17.395
+fat bits in an image editor.
+
+154
+00:08:17.395 --> 00:08:19.241
+Pixels aren't squares either,
+
+155
+00:08:19.241 --> 00:08:23.081
+they're samples of a 2-dimensional function space and so they're also,
+
+156
+00:08:23.081 --> 00:08:26.366
+conceptually, infinitely small points.
+
+157
+00:08:26.366 --> 00:08:28.500
+Practically, it's a real pain in the ass to see
+
+158
+00:08:28.500 --> 00:08:30.804
+or manipulate infinitely small anything.
+
+159
+00:08:30.804 --> 00:08:32.212
+So big squares it is.
+
+160
+00:08:32.212 --> 00:08:35.966
+Digital stairstep drawings are exactly the same thing.
+
+161
+00:08:35.966 --> 00:08:37.684
+It's just a convenient drawing.
+
+162
+00:08:37.684 --> 00:08:40.404
+The stairsteps aren't really there.
+
+163
+00:08:45.652 --> 00:08:48.233
+When we convert a digital signal back to analog,
+
+164
+00:08:48.233 --> 00:08:50.900
+the result is _also_ smooth regardless of the bit depth.
+
+165
+00:08:50.900 --> 00:08:53.193
+24 bits or 16 bits...
+
+166
+00:08:53.193 --> 00:08:54.196
+or 8 bits...
+
+167
+00:08:54.196 --> 00:08:55.486
+it doesn't matter.
+
+168
+00:08:55.486 --> 00:08:57.534
+So does that mean that the digital bit depth
+
+169
+00:08:57.534 --> 00:08:58.953
+makes no difference at all?
+
+170
+00:08:59.245 --> 00:09:00.521
+Of course not.
+
+171
+00:09:02.121 --> 00:09:06.046
+Channel 2 here is the same sine wave input,
+
+172
+00:09:06.046 --> 00:09:09.086
+but we quantize with dither down to eight bits.
+
+173
+00:09:09.086 --> 00:09:14.174
+On the scope, we still see a nice
+smooth sine wave on channel 2.
+
+174
+00:09:14.174 --> 00:09:18.014
+Look very close, and you'll also see a
+bit more noise.
+
+175
+00:09:18.014 --> 00:09:19.305
+That's a clue.
+
+176
+00:09:19.305 --> 00:09:21.273
+If we look at the spectrum of the signal...
+
+177
+00:09:22.889 --> 00:09:23.732
+aha!
+
+178
+00:09:23.732 --> 00:09:26.398
+Our sine wave is still there unaffected,
+
+179
+00:09:26.398 --> 00:09:28.490
+but the noise level of the eight-bit signal
+
+180
+00:09:28.490 --> 00:09:32.470
+on the second channel is much higher!
+
+181
+00:09:32.948 --> 00:09:36.148
+And that's the difference the number of bits makes.
+
+182
+00:09:36.148 --> 00:09:37.434
+That's it!
+
+183
+00:09:37.822 --> 00:09:39.956
+When we digitize a signal, first we sample it.
+
+184
+00:09:39.956 --> 00:09:42.366
+The sampling step is perfect; it loses nothing.
+
+185
+00:09:42.366 --> 00:09:45.626
+But then we quantize it,
+and quantization adds noise.
+
+186
+00:09:47.827 --> 00:09:50.793
+The number of bits determines how much noise
+
+187
+00:09:50.793 --> 00:09:52.569
+and so the level of the
+noise floor.
+
+188
+00:10:00.170 --> 00:10:03.646
+What does this dithered quantization noise sound like?
+
+189
+00:10:03.646 --> 00:10:06.012
+Let's listen to our eight-bit sine wave.
+
+190
+00:10:12.521 --> 00:10:15.273
+That may have been hard to hear anything but the tone.
+
+191
+00:10:15.273 --> 00:10:18.740
+Let's listen to just the noise after we notch out the sine wave
+
+192
+00:10:18.740 --> 00:10:21.683
+and then bring the gain up a bit because the noise is quiet.
+
+193
+00:10:32.009 --> 00:10:35.049
+Those of you who have used analog recording equipment
+
+194
+00:10:35.049 --> 00:10:36.670
+may have just thought to yourselves,
+
+195
+00:10:36.670 --> 00:10:40.382
+"My goodness! That sounds like tape hiss!"
+
+196
+00:10:40.382 --> 00:10:41.929
+Well, it doesn't just sound like tape hiss,
+
+197
+00:10:41.929 --> 00:10:43.433
+it acts like it too,
+
+198
+00:10:43.433 --> 00:10:45.225
+and if we use a gaussian dither
+
+199
+00:10:45.225 --> 00:10:47.646
+then it's mathematically equivalent in every way.
+
+200
+00:10:47.646 --> 00:10:49.225
+It _is_ tape hiss.
+
+201
+00:10:49.225 --> 00:10:51.774
+Intuitively, that means that we can measure tape hiss
+
+202
+00:10:51.774 --> 00:10:54.196
+and thus the noise floor of magnetic audio tape
+
+203
+00:10:54.196 --> 00:10:56.233
+in bits instead of decibels,
+
+204
+00:10:56.233 --> 00:10:59.902
+in order to put things in a digital perspective.
+
+205
+00:10:59.902 --> 00:11:03.028
+Compact cassettes...
+
+206
+00:11:03.028 --> 00:11:05.449
+for those of you who are old enough to remember them,
+
+207
+00:11:05.449 --> 00:11:09.161
+could reach as
+deep as nine bits in perfect conditions,
+
+208
+00:11:09.161 --> 00:11:11.209
+though five to six bits was more typical,
+
+209
+00:11:11.209 --> 00:11:13.876
+especially if it was a recording made on a tape deck.
+
+210
+00:11:13.876 --> 00:11:19.422
+That's right... your mix tapes were only about six bits
+deep... if you were lucky!
+
+211
+00:11:19.837 --> 00:11:22.345
+The very best professional open reel tape
+
+212
+00:11:22.345 --> 00:11:24.553
+used in studios could barely hit...
+
+213
+00:11:24.553 --> 00:11:26.473
+any guesses?...
+
+214
+00:11:26.473 --> 00:11:27.604
+13 bits
+
+215
+00:11:27.604 --> 00:11:28.980
+_with_ advanced noise reduction.
+
+216
+00:11:28.980 --> 00:11:32.062
+And that's why seeing 'DDD' on a Compact Disc
+
+217
+00:11:32.062 --> 00:11:35.208
+used to be such a big, high-end deal.
+
+218
+00:11:40.116 --> 00:11:42.825
+I keep saying that I'm quantizing with dither,
+
+219
+00:11:42.825 --> 00:11:44.734
+so what is dither exactly?
+
+220
+00:11:44.734 --> 00:11:47.284
+More importantly, what does it do?
+
+221
+00:11:47.284 --> 00:11:49.876
+The simple way to quantize a signal is to choose
+
+222
+00:11:49.876 --> 00:11:52.329
+the digital amplitude value closest
+
+223
+00:11:52.329 --> 00:11:54.377
+to the original analog amplitude.
+
+224
+00:11:54.377 --> 00:11:55.337
+Obvious, right?
+
+225
+00:11:55.337 --> 00:11:57.545
+Unfortunately, the exact noise you get
+
+226
+00:11:57.545 --> 00:11:59.220
+from this simple quantization scheme
+
+227
+00:11:59.220 --> 00:12:02.174
+depends somewhat on the input signal,
+
+228
+00:12:02.174 --> 00:12:04.596
+so we may get noise that's inconsistent,
+
+229
+00:12:04.596 --> 00:12:06.142
+or causes distortion,
+
+230
+00:12:06.142 --> 00:12:09.054
+or is undesirable in some other way.
+
+231
+00:12:09.054 --> 00:12:11.764
+Dither is specially-constructed noise that
+
+232
+00:12:11.764 --> 00:12:15.273
+substitutes for the noise produced by simple quantization.
+
+233
+00:12:15.273 --> 00:12:18.025
+Dither doesn't drown out or mask quantization noise,
+
+234
+00:12:18.025 --> 00:12:20.190
+it actually replaces it
+
+235
+00:12:20.190 --> 00:12:22.612
+with noise characteristics of our choosing
+
+236
+00:12:22.612 --> 00:12:24.794
+that aren't influenced by the input.
+
+237
+00:12:25.256 --> 00:12:27.081
+Let's _watch_ what dither does.
+
+238
+00:12:27.081 --> 00:12:30.078
+The signal generator has too much noise for this test
+
+239
+00:12:30.431 --> 00:12:33.161
+so we'll produce a mathematically
+
+240
+00:12:33.161 --> 00:12:34.782
+perfect sine wave with the ThinkPad
+
+241
+00:12:34.782 --> 00:12:38.205
+and quantize it to eight bits with dithering.
+
+242
+00:12:39.006 --> 00:12:41.342
+We see a nice sine wave on the waveform display
+
+243
+00:12:41.342 --> 00:12:43.452
+and output scope
+
+244
+00:12:44.222 --> 00:12:44.972
+and...
+
+245
+00:12:46.588 --> 00:12:49.375
+once the analog spectrum analyzer catches up...
+
+246
+00:12:50.713 --> 00:12:53.588
+a clean frequency peak with a uniform noise floor
+
+247
+00:12:56.864 --> 00:12:58.611
+on both spectral displays
+
+248
+00:12:58.611 --> 00:12:59.646
+just like before
+
+249
+00:12:59.646 --> 00:13:01.549
+Again, this is with dither.
+
+250
+00:13:02.196 --> 00:13:04.225
+Now I turn dithering off.
+
+251
+00:13:05.779 --> 00:13:07.913
+The quantization noise, that dither had spread out
+
+252
+00:13:07.913 --> 00:13:09.577
+into a nice, flat noise floor,
+
+253
+00:13:09.577 --> 00:13:12.286
+piles up into harmonic distortion peaks.
+
+254
+00:13:12.286 --> 00:13:16.030
+The noise floor is lower, but the level of distortion becomes nonzero,
+
+255
+00:13:16.030 --> 00:13:19.668
+and the distortion peaks sit higher than the dithering noise did.
+
+256
+00:13:19.668 --> 00:13:22.318
+At eight bits this effect is exaggerated.
+
+257
+00:13:22.488 --> 00:13:24.200
+At sixteen bits,
+
+258
+00:13:24.692 --> 00:13:25.929
+even without dither,
+
+259
+00:13:25.929 --> 00:13:28.308
+harmonic distortion is going to be so low
+
+260
+00:13:28.308 --> 00:13:30.708
+as to be completely inaudible.
+
+261
+00:13:30.708 --> 00:13:34.581
+Still, we can use dither to eliminate it completely
+
+262
+00:13:34.581 --> 00:13:36.489
+if we so choose.
+
+263
+00:13:37.642 --> 00:13:39.273
+Turning the dither off again for a moment,
+
+264
+00:13:40.934 --> 00:13:43.444
+you'll notice that the absolute level of distortion
+
+265
+00:13:43.444 --> 00:13:47.070
+from undithered quantization stays approximately constant
+
+266
+00:13:47.070 --> 00:13:49.033
+regardless of the input amplitude.
+
+267
+00:13:49.033 --> 00:13:51.998
+But when the signal level drops below a half a bit,
+
+268
+00:13:51.998 --> 00:13:54.036
+everything quantizes to zero.
+
+269
+00:13:54.036 --> 00:13:54.910
+In a sense,
+
+270
+00:13:54.910 --> 00:13:58.557
+everything quantizing to zero is just 100% distortion!
+
+271
+00:13:58.833 --> 00:14:01.588
+Dither eliminates this distortion too.
+
+272
+00:14:01.588 --> 00:14:03.599
+We reenable dither and...
+
+273
+00:14:03.599 --> 00:14:06.377
+there's our signal back at 1/4 bit,
+
+274
+00:14:06.377 --> 00:14:09.076
+with our nice flat noise floor.
+
+275
+00:14:09.630 --> 00:14:11.220
+The noise floor doesn't have to be flat.
+
+276
+00:14:11.220 --> 00:14:12.798
+Dither is noise of our choosing,
+
+277
+00:14:12.798 --> 00:14:15.006
+so let's choose a noise as inoffensive
+
+278
+00:14:15.006 --> 00:14:17.017
+and difficult to notice as possible.
+
+279
+00:14:18.142 --> 00:14:22.484
+Our hearing is most sensitive in the midrange from 2kHz to 4kHz,
+
+280
+00:14:22.484 --> 00:14:25.438
+so that's where background noise is going to be the most obvious.
+
+281
+00:14:25.438 --> 00:14:29.406
+We can shape dithering noise away from sensitive frequencies
+
+282
+00:14:29.406 --> 00:14:31.241
+to where hearing is less sensitive,
+
+283
+00:14:31.241 --> 00:14:33.910
+usually the highest frequencies.
+
+284
+00:14:34.249 --> 00:14:37.460
+16-bit dithering noise is normally much too quiet to hear at all,
+
+285
+00:14:37.460 --> 00:14:39.668
+but let's listen to our noise shaping example,
+
+286
+00:14:39.668 --> 00:14:42.234
+again with the gain brought way up...
+
+287
+00:14:56.020 --> 00:14:59.977
+Lastly, dithered quantization noise _is_ higher power overall
+
+288
+00:14:59.977 --> 00:15:04.276
+than undithered quantization noise even when it sounds quieter.
+
+289
+00:15:04.276 --> 00:15:07.902
+You can see that on a VU meter during passages of near-silence.
+
+290
+00:15:07.902 --> 00:15:10.537
+But dither isn't only an on or off choice.
+
+291
+00:15:10.537 --> 00:15:14.712
+We can reduce the dither's power to balance less noise against
+
+292
+00:15:14.712 --> 00:15:18.313
+a bit of distortion to minimize the overall effect.
+
+293
+00:15:19.605 --> 00:15:22.790
+We'll also modulate the input signal like this:
+
+294
+00:15:27.098 --> 00:15:30.206
+...to show how a varying input affects the quantization noise.
+
+295
+00:15:30.206 --> 00:15:33.289
+At full dithering power, the noise is uniform, constant,
+
+296
+00:15:33.289 --> 00:15:35.643
+and featureless just like we expect:
+
+297
+00:15:40.937 --> 00:15:42.772
+As we reduce the dither's power,
+
+298
+00:15:42.772 --> 00:15:46.356
+the input increasingly affects the amplitude and the character
+
+299
+00:15:46.356 --> 00:15:47.977
+of the quantization noise:
+
+300
+00:16:09.883 --> 00:16:13.844
+Shaped dither behaves similarly,
+
+301
+00:16:13.844 --> 00:16:16.553
+but noise shaping lends one more nice advantage.
+
+302
+00:16:16.553 --> 00:16:18.804
+To make a long story short, it can use
+
+303
+00:16:18.804 --> 00:16:20.937
+a somewhat lower dither power before the input
+
+304
+00:16:20.937 --> 00:16:23.662
+has as much effect on the output.
+
+305
+00:16:49.172 --> 00:16:51.508
+Despite all the time I just spent on dither,
+
+306
+00:16:51.508 --> 00:16:53.012
+we're talking about differences
+
+307
+00:16:53.012 --> 00:16:56.372
+that start 100 decibels below full scale.
+
+308
+00:16:56.372 --> 00:16:59.806
+Maybe if the CD had been 14 bits as originally designed,
+
+309
+00:16:59.806 --> 00:17:01.513
+dither _might_ be more important.
+
+310
+00:17:01.989 --> 00:17:02.644
+Maybe.
+
+311
+00:17:02.644 --> 00:17:05.438
+At 16 bits, really, it's mostly a wash.
+
+312
+00:17:05.438 --> 00:17:08.019
+You can think of dither as an insurance policy
+
+313
+00:17:08.019 --> 00:17:11.443
+that gives several extra decibels of dynamic range
+
+314
+00:17:11.443 --> 00:17:12.804
+just in case.
+
+315
+00:17:12.990 --> 00:17:14.196
+The simple fact is, though,
+
+316
+00:17:14.196 --> 00:17:16.361
+no one ever ruined a great recording
+
+317
+00:17:16.361 --> 00:17:19.182
+by not dithering the final master.
+
+318
+00:17:24.414 --> 00:17:25.790
+We've been using sine waves.
+
+319
+00:17:25.790 --> 00:17:28.254
+They're the obvious choice when what we want to see
+
+320
+00:17:28.254 --> 00:17:32.212
+is a system's behavior at a given isolated frequency.
+
+321
+00:17:32.212 --> 00:17:34.217
+Now let's look at something a bit more complex.
+
+322
+00:17:34.217 --> 00:17:35.923
+What should we expect to happen
+
+323
+00:17:35.923 --> 00:17:39.671
+when I change the input to a square wave...
+
+324
+00:17:42.718 --> 00:17:45.921
+The input scope confirms our 1kHz square wave.
+
+325
+00:17:45.921 --> 00:17:47.351
+The output scope shows..
+
+326
+00:17:48.614 --> 00:17:51.102
+Exactly what it should.
+
+327
+00:17:51.102 --> 00:17:53.900
+What is a square wave really?
+
+328
+00:17:54.654 --> 00:17:57.982
+Well, we can say it's a waveform that's some positive value
+
+329
+00:17:57.982 --> 00:18:00.788
+for half a cycle and then transitions instantaneously
+
+330
+00:18:00.788 --> 00:18:02.910
+to a negative value for the other half.
+
+331
+00:18:02.910 --> 00:18:05.076
+But that doesn't really tell us anything useful
+
+332
+00:18:05.076 --> 00:18:07.241
+about how this input
+
+333
+00:18:07.241 --> 00:18:09.378
+becomes this output.
+
+334
+00:18:10.132 --> 00:18:12.713
+Then we remember that any waveform
+
+335
+00:18:12.713 --> 00:18:15.508
+is also the sum of discrete frequencies,
+
+336
+00:18:15.508 --> 00:18:18.302
+and a square wave is a particularly simple sum
+
+337
+00:18:18.302 --> 00:18:19.636
+a fundamental and
+
+338
+00:18:19.636 --> 00:18:22.228
+an infinite series of odd harmonics.
+
+339
+00:18:22.228 --> 00:18:24.597
+Sum them all up, you get a square wave.
+
+340
+00:18:26.398 --> 00:18:27.433
+At first glance,
+
+341
+00:18:27.433 --> 00:18:29.225
+that doesn't seem very useful either.
+
+342
+00:18:29.225 --> 00:18:31.561
+You have to sum up an infinite number of harmonics
+
+343
+00:18:31.561 --> 00:18:33.108
+to get the answer.
+
+344
+00:18:33.108 --> 00:18:35.977
+Ah, but we don't have an infinite number of harmonics.
+
+345
+00:18:36.960 --> 00:18:39.902
+We're using a quite sharp anti-aliasing filter
+
+346
+00:18:39.902 --> 00:18:42.206
+that cuts off right above 20kHz,
+
+347
+00:18:42.206 --> 00:18:44.158
+so our signal is band-limited,
+
+348
+00:18:44.158 --> 00:18:46.421
+which means we get this:
+
+349
+00:18:52.500 --> 00:18:56.468
+..and that's exactly what we see on the output scope.
+
+350
+00:18:56.468 --> 00:18:59.550
+The rippling you see around sharp edges in a bandlimited signal
+
+351
+00:18:59.550 --> 00:19:00.926
+is called the Gibbs effect.
+
+352
+00:19:00.926 --> 00:19:04.137
+It happens whenever you slice off part of the frequency domain
+
+353
+00:19:04.137 --> 00:19:07.006
+in the middle of nonzero energy.
+
+354
+00:19:07.006 --> 00:19:09.854
+The usual rule of thumb you'll hear is the sharper the cutoff,
+
+355
+00:19:09.854 --> 00:19:11.188
+the stronger the rippling,
+
+356
+00:19:11.188 --> 00:19:12.777
+which is approximately true,
+
+357
+00:19:12.777 --> 00:19:14.900
+but we have to be careful how we think about it.
+
+358
+00:19:14.900 --> 00:19:15.774
+For example...
+
+359
+00:19:15.774 --> 00:19:19.529
+what would you expect our quite sharp anti-aliasing filter
+
+360
+00:19:19.529 --> 00:19:23.181
+to do if I run our signal through it a second time?
+
+361
+00:19:34.136 --> 00:19:37.588
+Aside from adding a few fractional cycles of delay,
+
+362
+00:19:37.588 --> 00:19:39.348
+the answer is...
+
+363
+00:19:39.348 --> 00:19:40.857
+nothing at all.
+
+364
+00:19:41.257 --> 00:19:43.302
+The signal is already bandlimited.
+
+365
+00:19:43.656 --> 00:19:46.590
+Bandlimiting it again doesn't do anything.
+
+366
+00:19:46.590 --> 00:19:50.686
+A second pass can't remove frequencies that we already removed.
+
+367
+00:19:52.070 --> 00:19:53.737
+And that's important.
+
+368
+00:19:53.737 --> 00:19:56.233
+People tend to think of the ripples as a kind of artifact
+
+369
+00:19:56.233 --> 00:19:59.945
+that's added by anti-aliasing and anti-imaging filters,
+
+370
+00:19:59.945 --> 00:20:01.737
+implying that the ripples get worse
+
+371
+00:20:01.737 --> 00:20:03.913
+each time the signal passes through.
+
+372
+00:20:03.913 --> 00:20:05.950
+We can see that in this case that didn't happen.
+
+373
+00:20:05.950 --> 00:20:09.492
+So was it really the filter that added the ripples the first time through?
+
+374
+00:20:09.492 --> 00:20:10.537
+No, not really.
+
+375
+00:20:10.537 --> 00:20:12.126
+It's a subtle distinction,
+
+376
+00:20:12.126 --> 00:20:15.252
+but Gibbs effect ripples aren't added by filters,
+
+377
+00:20:15.252 --> 00:20:18.836
+they're just part of what a bandlimited signal _is_.
+
+378
+00:20:18.836 --> 00:20:20.798
+Even if we synthetically construct
+
+379
+00:20:20.798 --> 00:20:23.508
+what looks like a perfect digital square wave,
+
+380
+00:20:23.508 --> 00:20:26.206
+it's still limited to the channel bandwidth.
+
+381
+00:20:26.206 --> 00:20:29.140
+Remember the stairstep representation is misleading.
+
+382
+00:20:29.140 --> 00:20:32.222
+What we really have here are instantaneous sample points,
+
+383
+00:20:32.222 --> 00:20:36.148
+and only one bandlimited signal fits those points.
+
+384
+00:20:36.148 --> 00:20:39.614
+All we did when we drew our apparently perfect square wave
+
+385
+00:20:39.614 --> 00:20:43.198
+was line up the sample points just right so it appeared
+
+386
+00:20:43.198 --> 00:20:47.785
+that there were no ripples if we played connect-the-dots.
+
+387
+00:20:47.785 --> 00:20:49.449
+But the original bandlimited signal,
+
+388
+00:20:49.449 --> 00:20:52.742
+complete with ripples, was still there.
+
+389
+00:20:54.004 --> 00:20:56.542
+And that leads us to one more important point.
+
+390
+00:20:56.542 --> 00:20:59.550
+You've probably heard that the timing precision of a digital signal
+
+391
+00:20:59.550 --> 00:21:02.409
+is limited by its sample rate; put another way,
+
+392
+00:21:02.409 --> 00:21:05.140
+that digital signals can't represent anything
+
+393
+00:21:05.140 --> 00:21:08.041
+that falls between the samples...
+
+394
+00:21:08.041 --> 00:21:11.422
+implying that impulses or fast attacks have to align
+
+395
+00:21:11.422 --> 00:21:14.473
+exactly with a sample, or the timing gets mangled...
+
+396
+00:21:14.473 --> 00:21:16.219
+or they just disappear.
+
+397
+00:21:16.711 --> 00:21:20.820
+At this point, we can easily see why that's wrong.
+
+398
+00:21:20.820 --> 00:21:23.742
+Again, our input signals are bandlimited.
+
+399
+00:21:23.742 --> 00:21:26.036
+And digital signals are samples,
+
+400
+00:21:26.036 --> 00:21:29.340
+not stairsteps, not 'connect-the-dots'.
+
+401
+00:21:31.572 --> 00:21:34.592
+We most certainly can, for example,
+
+402
+00:21:36.777 --> 00:21:39.337
+put the rising edge of our bandlimited square wave
+
+403
+00:21:39.337 --> 00:21:42.004
+anywhere we want between samples.
+
+404
+00:21:42.004 --> 00:21:44.354
+It's represented perfectly
+
+405
+00:21:47.508 --> 00:21:50.218
+and it's reconstructed perfectly.
+
+406
+00:22:04.620 --> 00:22:06.526
+Just like in the previous episode,
+
+407
+00:22:06.526 --> 00:22:08.393
+we've covered a broad range of topics,
+
+408
+00:22:08.393 --> 00:22:10.868
+and yet barely scratched the surface of each one.
+
+409
+00:22:10.868 --> 00:22:13.620
+If anything, my sins of omission are greater this time around...
+
+410
+00:22:13.620 --> 00:22:16.286
+but this is a good stopping point.
+
+411
+00:22:16.286 --> 00:22:17.833
+Or maybe, a good starting point.
+
+412
+00:22:17.833 --> 00:22:18.708
+Dig deeper.
+
+413
+00:22:18.708 --> 00:22:19.710
+Experiment.
+
+414
+00:22:19.710 --> 00:22:21.374
+I chose my demos very carefully
+
+415
+00:22:21.374 --> 00:22:23.668
+to be simple and give clear results.
+
+416
+00:22:23.668 --> 00:22:26.217
+You can reproduce every one of them on your own if you like.
+
+417
+00:22:26.217 --> 00:22:28.766
+But let's face it, sometimes we learn the most
+
+418
+00:22:28.766 --> 00:22:30.516
+about a spiffy toy by breaking it open
+
+419
+00:22:30.516 --> 00:22:32.553
+and studying all the pieces that fall out.
+
+420
+00:22:32.553 --> 00:22:35.230
+That's OK, we're engineers.
+
+421
+00:22:35.230 --> 00:22:36.350
+Play with the demo parameters,
+
+422
+00:22:36.350 --> 00:22:37.972
+hack up the code,
+
+423
+00:22:37.972 --> 00:22:39.774
+set up alternate experiments.
+
+424
+00:22:39.774 --> 00:22:40.692
+The source code for everything,
+
+425
+00:22:40.692 --> 00:22:42.398
+including the little pushbutton demo application,
+
+426
+00:22:42.398 --> 00:22:44.361
+is up at Xiph.Org.
+
+427
+00:22:44.361 --> 00:22:45.940
+In the course of experimentation,
+
+428
+00:22:45.940 --> 00:22:47.401
+you're likely to run into something
+
+429
+00:22:47.401 --> 00:22:49.950
+that you didn't expect and can't explain.
+
+430
+00:22:49.950 --> 00:22:51.198
+Don't worry!
+
+431
+00:22:51.198 --> 00:22:54.537
+My earlier snark aside, Wikipedia is fantastic for
+
+432
+00:22:54.537 --> 00:22:56.788
+exactly this kind of casual research.
+
+433
+00:22:56.788 --> 00:22:59.956
+If you're really serious about understanding signals,
+
+434
+00:22:59.956 --> 00:23:03.337
+several universities have advanced materials online,
+
+435
+00:23:03.337 --> 00:23:07.380
+such as the 6.003 and 6.007 Signals and Systems modules
+
+436
+00:23:07.380 --> 00:23:08.798
+at MIT OpenCourseWare.
+
+437
+00:23:08.798 --> 00:23:11.593
+And of course, there's always the community here at Xiph.Org.
+
+438
+00:23:12.792 --> 00:23:13.929
+Digging deeper or not,
+
+439
+00:23:13.929 --> 00:23:14.974
+I am out of coffee,
+
+440
+00:23:14.974 --> 00:23:16.436
+so, until next time,
+
+441
+00:23:16.436 --> 00:23:19.316
+happy hacking!
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--- websites/xiph.org/video/vid2.shtml (rev 0)
+++ websites/xiph.org/video/vid2.shtml 2013-02-25 23:17:42 UTC (rev 18828)
@@ -0,0 +1,182 @@
+<!--#include virtual="/ssi/videoheader.include" -->
+
+<title>Xiph.Org Video Presentations: Digital Show & Tell</title>
+
+<link rel="alternate" type="application/atom+xml"
+ title="Latest Videos Feed"
+ href="atom-feed.xml">
+
+<!--#include virtual="/common/xiphbar_video.shtml" -->
+
+<h1>Digital Show & Tell</h1>
+<div class="or"><a href="/video">(...up to the main video page)</a></div>
+
+<!--#include virtual="/ssi/navbar_video.include" -->
+
+<div class="vid_wrapper">
+ <div class="vid_control">
+ <div class="vid_border">
+ <div class="vid_sizer">
+
+ <!-- Firefox only added DOM support for callbacks in 4.0, so
+ they must go in the video tag -->
+ <!-- Attributes and source URL set via JA according to dropdpwn
+ selection -->
+ <video
+ controls
+ onplay="dimpage()"
+ onpause="undimpage()"
+ onended="undimpage()">
+
+ <!--#include virtual="fallback.include" -->
+
+ </video>
+ </div>
+ </div>
+
+ <!-- Subtitle overlay (driven by javascript in subtitiles.js) -->
+ <div class="srtwrapper">
+ <div class="baseline">Baseline</div>
+ <div class="srt"></div>
+ <div class="absbackground"></div>
+ </div>
+
+ <!-- Addiitional upper video control bar (driven by javascript in video.js) -->
+ <div class="vcwrapper">
+ <div class="vidcontrols">
+
+ <!-- where the video type text is filled in -->
+ <div class="vidtype">
+ </div>
+
+ <select class="video-select">
+ <option src-webm="https://media.xiph.org/monty/episode-02/02-Digital_Show_and_Tell-360p.webm"
+ src-ogg="http://downloads.xiph.org/video/Digital_Show_and_Tell-360p.ogv"
+ poster="vid2-poster-360.jpg"
+ video-width="640px"
+ video-height="360px"
+ subtitle-font-size="1.6em"
+ subtitle-bottom="40px">
+ 360p</option>
+ <option src-webm="https://media.xiph.org/monty/episode-02/02-Digital_Show_and_Tell-480p.webm"
+ src-ogg="http://downloads.xiph.org/video/Digital_Show_and_Tell-480p.ogv"
+ poster="vid2-poster-480.jpg"
+ video-width="848px"
+ video-height="480px"
+ subtitle-font-size="2em"
+ subtitle-bottom="40px">
+ 480p</option>
+ <option src-webm="https://media.xiph.org/monty/episode-02/02-Digital_Show_and_Tell-720p.webm"
+ src-ogg="http://downloads.xiph.org/video/Digital_Show_and_Tell-720p.ogv"
+ poster="vid2-poster-720.jpg"
+ video-width="1280px"
+ video-height="720px"
+ subtitle-font-size="3em"
+ subtitle-bottom="60px">
+ 720p</option>
+ </select>
+
+ <!-- chapter navigation -->
+ <select class="chapter-select">
+ <option>Chapter Selection</option>
+ <option timecode="0">title</option>
+ <option timecode="7.882">intro</option>
+ <option timecode="45.044">veritas ex machina</option>
+ <option timecode="216.424">stairsteps</option>
+ <option timecode="520.978">bit-depth</option>
+ <option timecode="695.820">dither</option>
+ <option timecode="1039.496">bandlimitation and timing</option>
+ <option timecode="1315.981">epilogue</option>
+ <option timecode="1398.772">end credits</option>
+ </select>
+
+ <!-- Subtitle selection (loaded from external file, not pulled
+ from media file) -->
+ <select class="srt-select">
+ <option>
+ Subtitles: Off</option>
+ <option file="vid2-en.vtt">
+ Subtitles: US English</option>
+ </select>
+
+ </div>
+
+ <!-- transparent dim background for addiitonal controls -->
+ <div class="absbackground"> </div>
+
+ <div style="clear: both; font-size: 0px;"></div>
+ </div>
+ </div>
+
+ <div style="clear: both; font-size: 0px;"></div>
+ <div class="vid_caption">
+
+ <p>
+ Continuing the "firehose" tradition of maximum information density, Xiph.Org's
+ second video on digital media explores multiple facets of digital
+ audio signals and how they <i>really</i> behave in the real
+ world.
+ </p>
+ <p>
+ Demonstrations of sampling, quantization, bit-depth, and dither
+ explore digital audio behavior on real audio equipment using
+ both modern digital analysis and vintage analog bench
+ equipment... just in case we can't trust those newfangled
+ digital gizmos. You can also download the source code for each
+ demo and try it all for yourself!
+ </p>
+
+ <h2><a href="http://wiki.xiph.org/Digital_Show_and_Tell/Episode_02">Discuss
+ and learn more: <i>"Digital Show & Tell"</i> the
+ Wiki-edition!</a></h2>
+
+ <p>Like the <a href="vid1.shtml">previous episode</a>, this video
+ moves fast and only glances on a number of rather important
+ topics, so we've set up
+ a <a href="http://wiki.xiph.org/Digital_Show_and_Tell/Episode_02">Wiki</a>
+ to get more information, ask questions, and debate.
+
+ <h2>Download this video!</h2>
+
+ <i>"Digital Show & Tell"</i> is distributed under
+ a <a href="http://creativecommons.org/licenses/by-sa/3.0/">Creative
+ Commons Attribution-ShareAlike (BY-SA)
+ license.</a>
+
+ <br/><br/>
+ <b>Download via HTTP</b>
+
+ <ul>
+ <li><b style="display: inline;"> WebM Format (Vorbis audio + VP8 video): </b><br>
+ <a href="http://downloads.xiph.org/video/Digital_Show_&_Tell-360p.webm">
+ 360p/500kbps (117MB)</a> |
+ <a href="http://downloads.xiph.org/video/Digital_Show_&_Tell-480p.webm">
+ 480p/1500kbps (294MB)</a> |
+ <a href="http://downloads.xiph.org/video/Digital_Show_&_Tell-720p.webm">
+ 720p/3500kbps (640MB)</a>
+
+ <li><b> Ogg Format (Vorbis audio / Theora video / Kate subtitles + Index): </b><br>
+ <a href="http://downloads.xiph.org/video/Digital_Show_&_Tell-360p.ogv">
+ 360p/500kbps (115MB)</a> |
+ <a href="http://downloads.xiph.org/video/Digital_Show_&_Tell-480p.ogv">
+ 480p/1500kbps (293MB)</a> |
+ <a href="http://downloads.xiph.org/video/Digital_Show_&_Tell-720p.ogv">
+ 720p/3500kbps (637MB)</a>
+ </ul>
+
+ <h3>Download subtitles</h3>
+
+ <ul>
+ <li><b> VTT format: </b><br>
+ <a href="vid2-en.vtt">
+ US English</a>
+ </ul>
+ <p>We welcome good, technical translations from the community!
+ Please submit translations in SRT, VTT or Ogg Kate format
+ to <a href="mailto:video at xiph.org">video at xiph.org</a>.
+
+ </div>
+</div>
+<!--#include virtual="/ssi/pagebottom.include" -->
+
+
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