[Flac-dev] Detecting lossy encodes

[Martin Leese]

Brian Willoughby <brianw at sounds.wa.com> wrote:

> On Jan 7, 2011, at 16:27, Declan Kelly wrote:
...
>> As the human hearing can't really tell direction with lower
>> frequencies,
>> it's not as essential. This same shortcut is why most movie "surround
>> sound" systems have only one sub bass channel.
>
> In this case, you have been misled by a common misconception in the
> consumer audio industry.
...
> The reason why most consumer electronics experts get this wrong is
> because of the standard techniques use in studio recording.  Most
> music is recorded as multiple channels, e.g., 16, that are each
> monophonic.  These channels are played back through a mixing console,
> and a simple pan pot is used to artificially place them in a
> location.  Because the pan pot only effects the volume, not the phase
> difference or time delay, this means that a studio recording is going
> to have no directionality at low frequencies.

The reality is more complicated than this.
When a pair of speakers are in front of the
listener and separated by 60 degrees or less,
because each ear hears both speakers,
low-frequency amplitude differences between
the speakers are converted to phase
differences between the ears.  This means
that pan-potted studio recordings have
directionality at low frequencies even though
the recording will contain only amplitude
differences.

Human hearing is discussed briefly in the
Ambisonic Surround Sound FAQ at:
http://members.tripod.com/martin_leese/Ambisonic/faq_latest.html#SECTION12

The conversion of low-frequency amplitude
differences at the speakers to phase between
the ears is nicely explained at:
http://uod-true-multi-channel-mixing.wikispaces.com/Blumlein+Stereo

As the article points out, Blumlein described
all this in his 1930s patent.


The reason why movie surround systems only
have one "sub bass channel" is also
complicated.  The key is to note that the
correct name for the .1 channel is the Low
Frequency Effects (LFE) channel.  It is there
only for special effects such as explosions and
dinosaur footsteps.  The channel's most
useful characteristic is therefore its extra
10 dB of headroom.

It is likely that humans localise very low
frequencies, but they probably use things like
chest cavities and not ears.  However, this is
usually irrelevant because, in most listening
rooms, localisation of low frequencies is
screwed up by standing waves.  And this is
why the typical listening room only needs one
sub-woofer.

As Brian pointed out, the other channels in
5.1 (or whatever) are all full range.  If every
system had effective bass management, the
LFE channel would be redundant for music
(with the exception of the cannons at the end
of the 1812 Overture).  Whether the
sub-woofer is redundant or not for music
depends on the bass handling of the main
speakers.

Regards,
Martin
-- 
Martin J Leese
E-mail: martin.leese  stanfordalumni.org
Web: http://members.tripod.com/martin_leese/