[NUT-devel] r21189 - trunk/DOCS/tech/nut.txt

[Ralph Giles]

On Fri, Nov 24, 2006 at 11:35:24PM +0100, Michael Niedermayer wrote:

> hmm, i see 3 possibilities for xiph codecs
> 1. store the metadata packet as is
> 2. dont store the metadata packet
> 3. store a dummy (empty) metadata packet

I would vote for 1 or 3.

I disagreed with doing 2 in the RTP draft, but didn't insist because the 
payload implementation already has to be codec-specific. For general 
container use like this, I think it's important to require the metadata 
packet where the codec requires it. This is less surprising for 
implementors.

You do still have to know how the vorbis headers are packed, but for 
playback there is still a difference between these two procedures:

# option 1 or 3
for header in global_headers:
  submit_packet_to_codec(header);

# option 2
header_number = 0;
for header in global_headers:
  submit_packet_to_codec(header)
  header_number += 1
  if (header_number == 1):
    header = construct_dummy_metadata_packet()
    submit_packet_to_codec(header)

I'd rather have to write the first one.

This imposes an overhead of (8 bytes + one packet overhead) on any 
header blob, assuming a minimal vorbis-style comment packet. I would
leave the encoder id string in the packet, so 

Therefore I'd propose, "The global headers MUST consist of the normal 
sequence of header packets required for codec initialization, in the 
order expected by the codec. An implementation MAY strip metadata and 
other redundant information not necessary for correct playback from the 
global headers to save space in the bitstream."

 -r