Time jitter in speakers (Roland werk )

Subject: Time jitter in speakers
From:    Roland werk  <R.W.M.Mathijssen(at)INAME.COM>
Date:    Thu, 25 Apr 2002 09:45:31 GMT

Hi,

Yesterday during a meeting about wireless communication I heard a comment I could not
quite understand. Perhaps someone from the list has some references or direct answers.

Problem: when sending data to two speakers in a stereo set-up one has to make sure that
the data that arrives at the speakers is converted into sound (decoding + ADC +
amplification etc.) synchronously. That is, both speakers should start at the same point
in time and each sample should be send out as sound at the same time during the whole
audio stream.

When one of the streams has jitter it is clear to me that this can be audible. I guess the
effect is similar to moving the speaker to the front and to the back. Next to this the
"mono" source probably also seems to shift from left to right between the speakers. This
seems okay to me, but just te be sure: IS THIS CORRECT?

When the audio streams start, I thought that a non-synchronous start of up to say a few
milliseconds would not be really noticable, since it seems similar to a speaker being
placed up to a few feet or something more to the front or back of the room (lots of people
have their speakers in a non-ideal position at home and do not complain). However, what
the presentation told was that the effect is much more noticable and that it seems much
more a left-right shift than a front-back shift. A difference in starting moment of less
than 1 msec (in my mind this is a speaker shift of 1 foot, 30 cm) would be quite
noticable. Figures of a few 100 usecs were mentioned. This is something I would never have
expected. Of course with headphones the effect will be very noticable (tens of
microseconds probably), but speakers were in my vision less susceptible to this problem.

Has anyone any papers, references about this effect? Or was the presentation not
completely correct perhaps? And are there any standard figures about allowable jitter /
asynchronicity in stereo speakers?

Cheers

Roland