[opus] Ogg Format

[Jean-Marc Valin]

On 05/09/2016 02:50 PM, Amit Ashara wrote:
> I am referring to the following file
> 
> https://en.wikipedia.org/wiki/File:Sample_of_%22Another_Day_in_Paradise%22.ogg

This is an Ogg/Vorbis file, not an Ogg/Opus file.

	Jean-Marc

> I opened the file in a HEX editor. I do not see the string OpusHead in
> the packet. It starts with Oggs. Also checking the occurrence of Oggs, I
> see that the first packet has BOS, the next all (except last) have 00
> (which is not defined in the RFC as continuation) and last one has EOS.
> 
> The first packet in this ogg file makes sense.
> 
> The second packet (I am taking a part of it)
> 
> 4f 67 67 53 = Oggs (THIS IS OK)
> 00 = Version (THIS IS OK)
> 00 = Header Type (WHY IS THIS 0)??
> 00 00 00 00 00 00 00 00 = Granule Position (THERE IS NO SPECIFIC
> INFORMATION)
> 7e 46 00 00 = Bit Stream Serial Number (THIS IS OK)
> 01 00 00 00 = Page Sequence Number (THIS IS OK)
> 89 ab 50 5e = Checksum (THIS IS OK)
> 10 = Page Segments is 16, so there would be 16 segments after this
> 2d ff ff ff ff ff ff ff ff ff ff ff ff ff ff c1 = Each Segment Size (IS
> THIS CORRECT)??
> 03 76 6f = First 3 data from Segment 0 indicated by the size 2d (IS THIS
> CORRECT INTERPRETATION)??
> 
> Regards
> Amit
> 
> On Mon, May 9, 2016 at 1:01 PM, Timothy B. Terriberry <tterribe at xiph.org
> <mailto:tterribe at xiph.org>> wrote:
> 
>     Amit Ashara wrote:
> 
>         First Packet shall have Header Type as BOS
>         All subsequent Packet (except last one_ shall have Header Type as
>         Continuation
>         Last Packet shall have Header Type as EOS
> 
>         Is this correct?
> 
> 
>     No, the header_type field is a property of a page, not a packet. In
>     it, the continuation flag is used when a single packet spans
>     multiple pages. This is required when a packet is larger than the
>     maximum size of a page (about 64 kB), but in most uses of Opus will
>     almost never be needed.
> 
>         The Page Segment shall have the value as 1 since only one
>         segment will
>         be available and the Segment Table will have the value 100
>         indicating
>         that the next 100 bytes are belonging to the same stream and is one
>         encoded frame
> 
>         Is this correct?
> 
> 
>     No. You are required to have at least three pages: one for the
>     identification header (OpusHead), which must contain only that
>     packet, one (or more) for the metadata header (OpusTags), which must
>     terminate the page it ends on, and then one more for your 20 ms
>     audio data packet.
> 
>     In the description, you also said that the final output was 100
>     halfwords (200 bytes). The value in the page segment table is the
>     number of bytes, not the number of halfwords.
> 
>         If the above is true then if the Page Segment has the value 20,
>         then the
>         segment table shall look something like (assuming 100 half words
>         is what
>         is encoded evry time)
> 
>         100, Data.... 20 times before the next Page Segment is started!!!
> 
> 
>     No. The segment table contains the lacing values for all packets on
>     the page, followed by the data for all packets. So it would look
>     more like:
> 
>     20 (number_page_segments) 100, 100, 100 ... (20 times, the
>     segment_table), data, data, data, ... (again, 20 times).
> 
>     It may be easier for you to work from some examples. I'd recommend
>     looking at the code in opus-tools:
>     <https://git.xiph.org/?p=opus-tools.git>. This gets all of these
>     details correct.
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> 
> 
> 
> 
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