Since I have nothing better to do with my life, I’ve decided to add a rudimentary support for encoding into RealMedia. I’ve written a somewhat working RMVB muxer (it lacks logical stream support that is used to describe the streaming capabilities, it also has some quirks in audio and video support but it seems to produce something that other players understand) and now I’ve made a Cook audio encoder as well.
Somebody who knows me knows that I fail spectacularly at writing non-trivial lossy audio encoders but luckily here we have a format which even I can’t botch much. It is based on parametric bit-allocation derived from band energies. So all it takes is to perform slightly modified MDCT, calculate band energies, convert them to scale factors, perform bit allocation based on them, pack band contents depending on band categories and adjust the categories until all bands fit the frame. All of these steps are well-defined (including the order in which bands should be adjusted) so making it all work is rather trivial. But what about determining the frame size and coupling mode?
As it turns out, RealMedia supports only certain codec profiles (or “flavors” in its terminology), so Cook has about 32 different flavours defined for different combinations of sample rates, number of channels and bitrates. And each flavour has an internally bitrate parameters (frame size and which coupling parameters to use) for each channel pair so you just pick a fitting profile and go on with it. In theory it’s possible to add a custom profile but it’s not worth the hassle IMO.
And now here are some fun facts about it. Apparently there are three internal revisions of the codec: the original version for RealMedia G2, version 2 for RealMedia 8 and multichannel encoder (introduced in RealMedia 10, when they’ve switched to AAC already). Version 2 is the one supporting joint-stereo coding. The codec is based on G.722.1 (the predecessor of CELT even if Xiph folks keep denying that) but, because Cook frames are 256-1024 samples instead of fixed 320-sample frames, they’ve introduced gains to adjust better for volume changes inside the frame (but I haven’t bothered implementing that part). That is probably the biggest principal change in the format (not counting the different frame sizes and joint stereo support in the version 2). And finally I should probably mention that there are some flavours with the same bitrate that differ by the frequency range and where the joint stereo part starts (some of those are called “high response music” whatever that means).
Time to move to the video encoder…