[Ffmpeg-devel] fix ac3 parser with realaudio

[Justin Ruggles]

Michael Niedermayer wrote:
> Hi
> 
> On Thu, Feb 08, 2007 at 06:50:22PM -0500, Justin Ruggles wrote:
> 
>>Hi,
>>
>>It seems that when the AC-3 parser was modified to add E-AC-3 support,
>>it stopped working with RealAudio/DolbyNet.  Below is a patch to fix it.
> 
> [...]
> 
>>@@ -667,7 +667,7 @@
>>         return 0;
>> 
>>     bsid = show_bits_long(&bits, 29) & 0x1f;
>>-    if(bsid <= 8) {             /* Normal AC-3 */
>>+    if(bsid <= 10) {             /* Normal AC-3 */
>>         skip_bits(&bits, 16);       /* crc */
>>         fscod = get_bits(&bits, 2);
>>         frmsizecod = get_bits(&bits, 6);
>>@@ -686,13 +686,14 @@
>>             skip_bits(&bits, 2);    /* dsurmod */
>>         lfeon = get_bits1(&bits);
>> 
>>-        *sample_rate = ac3_sample_rates[fscod];
>>-        *bit_rate = ac3_bitrates[frmsizecod] * 1000;
>>+        halfratecod = bsid - 8;
>>+        *sample_rate = ac3_sample_rates[fscod] >> halfratecod;
>>+        *bit_rate = (ac3_bitrates[frmsizecod] * 1000) >> halfratecod;
> 
> 
> this doesnt look correct, what is with bsid<8 ?

from A/52A:
"Values of bsid smaller than 8 will be used for versions of AC-3 which
implement subsets of the version 8 syntax. Decoders which can decode
version 8 will thus be able to decode version numbers less than 8."

> 
> 
>>         *channels = ac3_channels[acmod] + lfeon;
>>         *samples = 6 * 256;
>> 
>>         return ac3_frame_sizes[frmsizecod][fscod] * 2;
>>-    } else if (bsid >= 10 && bsid <= 16) { /* Enhanced AC-3 */
>>+    } else if (bsid > 10 && bsid <= 16) { /* Enhanced AC-3 */
> 
> 
> is this correct for E-AC3?

from A/52B:
"Values of bsid smaller than 16 and greater than 10 will be used for
versions of E-AC-3 which are backwards compatible with version 16
decoders. Decoders which can decode version 16 will thus be able to
decode version numbers less than 16 and greater than 10."