Axel WOLF 1MS, Руководство пользователя, Страница: 202 / 229

TECHNICAL APPENDIX
| TECHNICAL APPENDIX 202
Reload the web page (to be sure the gprsMode is activated) and push the play button.
In GPRS mode if one player is in listening mode the page refresh rate will be only one every six second.
In this condition the average bit rate (data + 1 stream) should be about 32 Kbit/sec.
We have focused on the point to give the best performances possible at very low bit rates (lower than 40 Kbits/sec). The
encoder has a strong VBR characterization (for example at 28Kbit/sec the instant bit rate sweep from 8 to 40 Kbits/sec,
with an average always less or equal 28kbytes. With a muted or simple audio (like a test tone or silence) the bit rate
drops down to 8 Kbits/sec, instead with voice or music we have an average value of 28Kbit.
The IceCast server have a 48Kbytes buffer for every stream and instead every client player (VLC and so on) have a
different and programmable buffer size (some time expressed in second, but often expressed in Kbytes).
In the instant of the connection, the delay line is expressed in Kbytes, so the same 48Kbytes of buffer can be 14 sec with
normal music or 48 seconds in an extreme case of silenced signal.
With a 128 Kbit/sec (a more classic bit rate) we have a delay between 3 to 5 sec. The player creates another buffer
locally (usually 64 Kbytes, so the final delay is a complex mixed situation about 20 seconds in the 28Kbit/s example and
about 4 seconds in a 128kbits/s case).
So with a real time player is very difficult to estimate the buffer delay, but a recorder can (partially) compensate it.
The Home page players are quite simple and are intended only for occasional check tests . The behavior can be little
different in Firefox or Chrome. The peak meters will be visible again in future.
In the next months will be available the OPUS codec and the VORBIS codec will be improved for the bit rates over 48
Kbits/s (full sample rate mode). The VORBIS quality at very low bit rates (under 48 Kbits/s) instead must be considered
definitive. By the way the OPUS codec will give better performance only in the middle rate zone (40Kbits/s-96kbits/s) ,
over 96Kbits the audio quality will be comparable.