[Mp4-tech] [Audio] Erroneous bit streams...

[Umang Garg]

Guha, Sris
While HEAAC for broadcast environments mandates the CRC check in ADTS
format, it usage appears to be quite redundant with little practical
advantage. It existence may have more to do with legacy reasons. 
The redundancy arises from the fact that the CRC validation of the
bitstream cannot happen untill the stream has been parsed upto a specified
number of bits. If the bitstream is corrupted in the 'right' places then it
is possible that the parsing process can go haywire. This implies that you
may not interpret the bits correctly and the parsing may take a completely
different control flow path. It is a chicken-and-egg situation.
Your best bet lies in the ability to handle serious corruption issues at
network level. Minor bitstream distortions can then be handled inside the
decoder by having a robust error handling mechanism along with CRC check.
Regards,
Umang 
  _____  
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of %Sriskanthan
Nadarajah
Sent: Monday, May 22, 2024 8:20 PM
To: guhapriya ganapathy; mp4-tech lists.mpegif.org
Subject: RE: [Mp4-tech] [Audio] Erroneous bit streams...
Hi Guha Priya.
The CRC should be checked and error passed before passing  the stream to the
Codec. 
Sris
WINUX Interfacing Technology Pte. Ltd.
Tele: (65) 6794-3069 H/P 92763116
Com. Reg. No: 200303307W
www.winux-it.com
  _____  
From: mp4-tech-bounces lists.mpegif.org on behalf of guhapriya ganapathy
Sent: Mon 5/22/2006 1:45 PM
To: mp4-tech lists.mpegif.org
Subject: RE: [Mp4-tech] [Audio] Erroneous bit streams...
Hi Umang,
In case of a packet loss, it is fine. We may have to make our own decisions.
Coming to bitstream corruption, what is the probability of the corrupted
bitstream reaching the codec? When a corruption is detected at the network
layer by the CRC, is it being dropped? If so, what is the need for CRC in
the codec?
Regards,
Guha Priya.
Umang Garg <ugarg neomagic.com> wrote:
Hello Guha, 
In broadcast environment you may encounter packet loss and stream corruption
as two primary errors. 
For HEAAC:
Stream corruption may be checked via CRC. ADTS format has a CRC check along
with a separate CRC check for SBR decoding.
Similarly, packet loss may also have to be handled gracefully within the
decoder. In case of a loss of frame(s) the simplest technique is insertion
of a silence frame (to maintain synchronization) but a more appropriate
technique would be gradual muting.  As far as I know, the HEAAC standard
does not say much about packet loss in general or the muting technique in
particular. You may have to make your own decisions on that.
Regards,
Umang  
  _____  
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of guhapriya ganapathy
Sent: Thursday, May 18, 2024 5:19 PM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] [Audio] Erroneous bit streams...
Dear all,
Suppose you have an audio codec which is a part of a broadcast system (any
system for that matter). What are the possible errors that might creep in
the bitstream when it reaches the decoder? (There will be CRC check for each
packet in the network layer, but I am concerned about the bitstream when it
reaches the decoder.. what are the chances that the bit stream is
corrupted?) 
Taking AAC HE as an example, Coding Technologies provides erroneous
bitstreams for SBR (although SBR has CRC check) as a part of its
certification package (wherein for example one bit in every third frame is
corrupted - a random corruption!), whereas ISO does not(?) provide erroneous
bitstreams. Of what value are erroneous bitstreams to the decoder? Is there
a value add in generating erroneous bitstreams for decoders? If so, what is
the approach so that it matches what happens in the real time?
Regards,
Guha Priya.
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