One of the criticisms often made of MAME / MESS’s CHD format is that it doesn’t actually provide very efficient compression, especially when it comes to CD AUDIO data. I’ve had a number of people ask me if I can look into improving this, especially when you consider that in with the current format a complete Saturn set is almost 1TB, with a large portion of that being AUDIO.
The reason it’s inefficient is because it’s using zlib’s inflate algorithm for the blocks, blocks which are rather small to ensure that data is decompressed quickly. While this is fine for DATA (it’s the same thing that ZIP files use) it’s absolutely hopeless for AUDIO.
There are dedicated audio lossless compressors out there, FLAC is a popular one.
I’ve spent the last 4-5 days solid integrating support for this into the MAME / MESS tree, and extending the CHD format to not only support it’s native blocks (hunks), but also reference to embedded streams via ‘virtual hunks’ which point at a stream, and allow the actual FLAC codec to do the seeking and decoding work for this.
By doing this I can achieve a good level of compression with FLAC, far better than trying to split it into CHD hunks due to the lower overhead, and improved ability of the compression algorithms to predict how data best compresses. I also still get good speed decoding, as the FLAC format is designed to be quick to seek, and has built in seektable support of it’s own which I’m levering.
I have to say FLAC is an absolute joy to work with, the API does everything you can expect, the documentation is great, and it’s very good at letting you know if something is wrong. (the only issue I had with the documentation / API was with the seektables, whereby calling things in the wrong order / wrong time during encoding could cause data to be overwritten without throwing an error)
I’ve also added support to the MAME SAMPLE interface to playback files from FLAC sources, this should allow the recently dumped tape loops to be compressed much better than they are now (they’re uncompressed PCM .wav files)
The other possibilities for this are endless, -wavwrite could also output FLAC data if support was added, MESS could potentially load cassette based software from FLAC images. It’s an incredibly useful codec to have around.
I’ve uploaded my first pass of this code
Here (link offline for the time being, there is definitely still an error). This should be considered ALPHA SOFTWARE and I won’t be held responsible if you end up destroying your CHDs with it. I’m currently in the process of batch converting many images and haven’t found a broken case yet, but still, it’s in testing. While I’m happy with the current format extensions and CHD format created it could change in a final version, you have been warned.
This code has been submitted to R.Belmont, who is currently making some portability fixes. FLAC is designed to be portable, so this shouldn’t be too much of a problem, so fingers crossed it can be sorted out soon.
Usage is simple, I’ve added an additional -createcdflac commandline option which will use the FLAC routines when compressing AUDIO. If you already know how to use CHDMAN then it’s simple enough.
Have fun :-)