[Speex-dev] How does the jitter buffer "catch up"?

Thorvald Natvig speex at natvig.com
Sun Sep 18 18:22:09 PDT 2005

>> (PS, if you do use threads, protect speex_jitter_put/get with a mutex
>> (CRITICAL_SECTION I believe they're called in Win32Speak) -- calling put
>> and get at the exact same time from different threads leads to "features")
> I've never tested this, but I designed the jitter buffer to work from
> two threads even without using a mutex. This would work as long as there
> is one thread doing the "put" and one thread doing the "get".

Err, unless I'm totally wrong, there are a few race conditions.

Assume the buffer is full of packets newer than the current pointer, and 
one that is at the current pointer.

get and put start at the same time.

get will find the correct buffer index. Now, just after it finds it's 
index, assume we switch to the put thread.

Put needs to put a new packet in, so discards the oldest, which is the 
current packet, and replaces it with the new one (let's call it newest).

get now starts decoding, but the index it found now points to the newest 
packet, which it will decode and remove from the buffer.

Granted, it's not terribly likely to happen, and the jitter-buffer as a 
whole has no crashbugs regarding two-thread access, but it will sound a 
bit odd if this should ever happen :)

>> As for detecting outages, a hack I use is to check jitter->valid_bits. If
>> it's set, we decoded "something", if it's not, we're interpolating
>> something which may not sound that good so feed the soundcard 20ms of
>> silence instead.
> Why would you do that. The idea of interpolating a frame is exactly to
> get better quality than just putting zeros.

Actually, I oversimplified a bit. I check if valid_bits has been zero for 
the last 4 frames or more, because once you interpolate more than 100ms 
from the last known state, you end up with some weird 
blipp-blopp-blooiiing sound. Actually it reminds me of the ambient sound 
of weird aliens in bad 50s scifi movies. At that point, silence is much 
better :)

More information about the Speex-dev mailing list