[M4IF Technotes] Switching between different bit-rates

[Herbert Thoma]

Dear Christian,
Ralph asked me to answer your questions.
There are basically two possibilities for switching between different
bitrates: single layer switching and scalable coding. Both is
possible for audio and video.
For single layer switching you need to encode your streams at different
bitrates. The client (player) sends information about the transmission
conditions to the server which chooses which bitrate to send. Here you
obviously need a backchannel. In the case of video, switching is only
possible at I frames. (the new MPEG-4 AVC/H.264 standard includes
mechanisms called SI and SP frames that allow switching even at non
I frames).
For sclable coding you encode your bitstream in a base layer and one or
more enhancement layers. The server sends all layers. The client can
produce an output if it gets only the base layer. If the client also
gets the enhancement layer(s) the quality will increase. Here you don't
need a backchannel and a quality of service (QoS) enabled network can
drop enhancement layer(s) automatically.
If the switching requires user interaction depends on the system design,
but I think it should usually be invisible. However, different users
may have different preferences, e.g. some may prefer higher spatial
resolution others higher framerate or fewer artifacts.
 Herbert.
Christian Kloeckner schrieb:
> 
> Dear Ralph,
> 
> is there a difference between video and audio concerning switching between
> different bit-rates?
> Will this switching require user interaction or will it be invisible for the
> user like Real's Surestream?
> 
> Thank You,
> Christian
> 
> >Dear Lucia,
> >
> >are you talking about video or audio?
> >
> >Anyway, a scalable system would be the first choice in your case. You would
> >encode a first layer with e.g. 32 kbit/s and a second (enhancement) layer
> with
> >again 32 kbit/s. Depending on the channel capacity, you might than transmit
> both
> >layers or only the first one (both layers are stored in separate
> ElementaryStreams).
> >
> >Best regards,
> >
> >Ralph
> 
> >> Is there some standardization on how to switch among files with different
> >> encoding rates?
> >> Is there a reference software that allows the switching between the two
> >>  files?
> >>
> >> I guess, if something exist it would be in the System part, Am i right?
> >>
> >> Thanks in advantage for your answer.
> >>
> >> Lucia Valentini
> ____________________________________________
> Christian Klöckner
> WDR, Technologie-Referat
> 50600 Köln, Germany
> 
> Tel: +49 (0) 221 220 9278
> Fax: +49 (0) 221 220 3167
> mailto: christian.kloeckner   wdr.de
> PGP: Public Key @ http://pgp.mit.edu
> ___________________________________________
> 
> _______________________________________________
> Technotes mailing list
> Technotes   lists.m4if.org
> http://lists.m4if.org/mailman/listinfo/technotes

-- 
Herbert Thoma
FhG-IIS A, Studio Department
Am Wolfsmantel 33, 91058 Erlangen, Germany
Phone: +49-9131-776-323
Fax:   +49-9131-776-399
email: tma   iis.fhg.de
www: http://www.iis.fhg.de/