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Forums > C64 Coding > Sample audio on C64, how to do a better conversion?
2020-07-17 08:00
zzarko

Registered: Feb 2003
Posts: 30
Sample audio on C64, how to do a better conversion?

Hi all!

Recently I have found about c64kernel and a movie player based on that code that uses 16MB REU as a storage. (https://github.com/vbguyny/c64kernel). Movie is based on Koala image sequences, and audio is 4bit samples, where playing routine is based on codebase64 code (https://codebase64.org/doku.php?id=base%3Anmi_sample_player) There is a manual that describes how to make a movie at
https://github.com/vbguyny/c64kernel/blob/master/doc/HowToMakeA..

Anyway, I wanted to make an automatic conversion from a movie file to format for that player and have come up with this Bash script:
https://bitbucket.org/zzarko/c64movie

For audio conversion I did this: converted audio to 8-bit unsigned raw format, and then extracted higher 4 bits from every byte. The result isn't bad, but it is somewhat worse than what "Wav to digi" program can produce (Wav to Digi). So, my question is what could I do to make audio conversion better?

I know that there are more advanced audio and video player routines by magicians around here (I enjoy watching their work!), but nevertheless, if you have some advice, please tell.

One of conversions I did this way:
https://www.youtube.com/watch?v=LN3y1koqi3A
2020-07-17 12:20
tlr

Registered: Sep 2003
Posts: 1321
The steps of $d418 volume changes aren't equal in size (more or less exponential). Just taking the upper 4 bits will give you some unwanted distortion. You should probably compensate for that to improve the fidelity.

More modern playback strategies will allow you to do 8-bit (linear) playback though.
2020-07-17 13:22
Krill

Registered: Apr 2002
Posts: 1493
Quoting tlr
The steps of $d418 volume changes aren't equal in size (more or less exponential). Just taking the upper 4 bits will give you some unwanted distortion. You should probably compensate for that to improve the fidelity.
This is true for 6581 oldsid, but not for 8580 newsid, right?

From https://sourceforge.net/p/sidplay-residfp/code/HEAD/tree/trunk/..
    // Non-linearity parameter, 8580 DACs are perfectly linear
    const double _2R_div_R = chipModel == MOS6581 ? 2.20 : 2.00;
That said, i'd really like to have a simple 16-values table with the non-linear curve. =)
2020-07-17 14:49
tlr

Registered: Sep 2003
Posts: 1321
Quoting Krill
Quoting tlr
The steps of $d418 volume changes aren't equal in size (more or less exponential). Just taking the upper 4 bits will give you some unwanted distortion. You should probably compensate for that to improve the fidelity.
This is true for 6581 oldsid, but not for 8580 newsid, right?

From https://sourceforge.net/p/sidplay-residfp/code/HEAD/tree/trunk/..
    // Non-linearity parameter, 8580 DACs are perfectly linear
    const double _2R_div_R = chipModel == MOS6581 ? 2.20 : 2.00;

Nah, that's a different set of DACs. The volume is handled using a 4-bit MDAC. In resid that's calculated in FilterModelConfig/FilterModelConfig8580 (returned by getGain() ).
Though the volume sample method requires some trickery to function on 8580 so it's not ideal there.

I admit to having no experience of 8580 on real HW so I can't really tell if the curve differs. :)
2020-07-17 14:56
Groepaz

Registered: Dec 2001
Posts: 9358
On real Hardware the curve also changes depending on Filter settings and currently playing voices. Also the impulse response is probably not quite symmetric either... so whatever conversion someone comes up with, it probably must also be tweaked to the player that plays it.
2020-07-17 15:01
tlr

Registered: Sep 2003
Posts: 1321
Interestingly the 6581 datasheet claims "Bits 0-3 (VOL0-VOL3) select 1 of 16 overall volume levels for the final composite audio output. The output volume levels range from no output (0) to maximum volume (15 or $F) in 16 linear steps."
From what I recall it doesn't try to be linear but I might be wrong. Didn't measure it myself.
2020-07-17 15:09
Groepaz

Registered: Dec 2001
Posts: 9358
The volume level isnt 1:1 related to the "sample value", its a bit less trivial than that :=)
2020-07-17 20:39
Krill

Registered: Apr 2002
Posts: 1493
Okay, but there should be some default non-linear mapping which is a sane compromise for most tunes on most SIDs. Anyone? :)
2020-07-17 21:06
Mixer

Registered: Apr 2008
Posts: 328
Mahoneys Music Run/Stop technical details pdf should have the data IIRC.
2020-07-17 21:59
Durandal

Registered: May 2006
Posts: 29
The Python conversion code we're using for streaming on the Turbo56K BBS, while not specifically taking into account any non linearity of the SID, it does a decent job in most cases.

This code accepts most audio file formats

import librosa
import numpy

y, sr = librosa.load(filename, samplerate, True) #load file

numpy.clip(y, -1, 1, y) #Clip samples
norm = librosa.mu_compress(y, mu=15, quantize=True) #mu-compress and quantize to 16 different values
norm = norm + 8 #offset the samples values
									
bin8 = numpy.uint8(norm) #convert to 8-bit unsigned


bin8 contains 4-bit sample data in the lower nibble of each byte.
2020-07-17 23:51
Krill

Registered: Apr 2002
Posts: 1493
Quoting Durandal
[...]not specifically taking into account any non linearity of the SID[...]
[...]mu_compress[...]
Okay, so my takeaway is that applying μ-law is better than using a linear mapping, no matter if specifically tweaked for SID or not.
2020-07-18 00:07
Groepaz

Registered: Dec 2001
Posts: 9358
My guess would be its just the implied lowpass that "does the job" in this case
2020-07-18 00:17
Krill

Registered: Apr 2002
Posts: 1493
Hmm, lowpass or not is mostly a function of catering to a specific sample frequency (downsampling in this case), not reducing bit depth, i thought?
2020-07-18 00:36
Groepaz

Registered: Dec 2001
Posts: 9358
Sure, that... and perhaps a bit of log compression too, which µ-law basically is.
2020-07-18 23:49
zzarko

Registered: Feb 2003
Posts: 30
Thank you all for answers, and especially to Durandal, his conversion gives better results than mine.

I would like to ask Durandal would it be OK to use his conversion code as part of my script, with given credits of course?
2020-07-19 00:24
Durandal

Registered: May 2006
Posts: 29
Sure you can use it, I wouldn't have posted it otherwise ;)
2020-07-29 22:18
zzarko

Registered: Feb 2003
Posts: 30
Thank you all for the help! Author of VBGuyNY C64 Kernel also made some modifications to the player code, now video loops when it finishes and code is more stable. Conversion script was also updated and cleaned.

Also, all this has made one of my dreams come true - State of the Art on C64 :)

https://www.youtube.com/watch?v=QAf5JsIje2Y
2020-07-30 04:46
Kruthers

Registered: Jul 2016
Posts: 21
Downloaded the examples and OMG! Thought I was the only who knew about Sledge Hammer...
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