Here are main differences between TrueSpeech and G.723:
1) One bitrate (8 Kb/s) vs. two bitrates (6.3 and 5.3 Kb/s)
2) Lower complexity (TrueSpeech uses 8-point LPC, G.723 – 10-point LPC)
Frame structure is also differs, TrueSpeech packs everything into 32 byte frame where some values are spread between different doublewords (3d bit is in one dword, 2nd is in another one).
DSP Group TrueSpeech audio codec is rumored to be a relative to ITU G.723. That may be partly true: DSP Group took part in developing G.723, but if you study recommendation G.723, you’ll see that there are two methods used in this rec., and they differ.
Studies showed that TrueSpeech may be one of predecessors of G.723 – it is simplier, needs higher bitrate than G.723 and such. But I hope to use some ideas to reconstruct TrueSpeech decoder using methods from G.723.
My TrueMotion 2 decoder is almost complete. It just has some problems with chroma decoding in intraframes. I suspect this is because of some rounding errors and such. Original TM2 decoder used to store chroma in pairs – one 4 double word for 16-bit Cr and Cb and all calculations were done on those pairs.
For now, I’ll leave this decoder for some time and will post pieces of information about other codecs.
TM2 frame is divided into blocks 4×4 and YUV420 colorspace is used, so for each 4×4 blocks of luminance there is two 2×2 blocks of chroma.
There are 7 types of blocks:
Every intraframe block (first four types in the list) uses such logic:
LAST 4 ELEMENTS
| | | |
V V V V
D0 -> +d00 +d10 +d20 +d30 -> D0
D1 -> +d01 +d11 +d21 +d31 -> D1
D2 -> +d02 +d12 +d22 +d32 -> D2
D3 -> +d03 +d13 +d23 +d33 -> D2
| | | |
V V V V
LAST 4 ELEMENTS
It means that every block use last 4 pixels from top block as reference and deltas D (differences between right pixel in this line and previous line) from left neighbour.
In case of low-res chroma or luma last elements and deltas are modified before processing, they are replaced with their average values.
For interframe blocks there is one additional operation – to calculate new deltas and last values from block data after opying is done.
TM2 differs from many modern and not modern codecs in aspect it uses different streams for different type of data – look at the scheme below:
[Global Header]
[Stream 0 - High-res chroma values]
[Stream 1 - Low-res chroma values]
[Stream 2 - High-res luminance values]
[Stream 3 - Low-res luminance values]
[Stream 4 - Update deltas values]
[Stream 5 - Motion vectors]
[Stream 6 - Block types]
Global header can be one of two types: old header type – all values are stored in 4-byte or 2-byte integers and new header type – header fields are stored in bitstream (9 bits for width and the same for height, 31 bits for flags, etc.).
One thing worth to mention – a mixture of big- and little-endian 4-byte integers in header values and use big-endian dwords in bitstream output.
Streams are huffman-coded array of values (called ‘tokens’) and consist of header, non-compulsory delta table (in this case tokens are just indices in that table), compressed Huffman tree and compressed tokens. Stream also can be empty (in case of intra-frames – that streams 4 and 5).
Decoder must decompress all these streams into memory and read tokens from one or another stream. Decoding process will be described in Part 2.
Duck Corp. (now On2) released GPLed source of their solution VPVision (capturing software for Windows), and it contains sources of their two codecs – TrueMotion 1 and TrueMotion 2 (predecessors of VP3-7 family).
TrueMotion 1 decoder is already present in FFmpeg, now it’s time to add TrueMotion 2.
The bad thing is that codec source code is poorly-documented and optimised for performance (optimisation is a form of obfuscation and vice-versa – look at The International Obfuscated C Code Contest for examples).
Welcome to new BLOG dedicated to RE’ing of codecs. I noted, that posting a description of codecs helps very much in process of their RE’ing. This will be the place to do it.