Amiga Sound Quality

[xArtx]

Hi Guys,

For instances where an emu can handle the audio,
why does an emulated Amiga sound like a real Amiga?

Do you disagree that it does?

Why can't something be applied to modern audio to do whatever is good
about it to modern audio, or audio samples?

[daxb]

Amiga 8 bit audio data have to go through pc audio hardware (soundcard, onboard). So the quality is similar to original. Amiga 16 bit audio data (AHI, ...?) should be as similar good as the pc audio hardware can provide.

There might be possibilities to change original 8bit audio data using pc audio filter on the fly but the quality won`t increase much. You know interpolation is interpolation (guessing). Toni can tell you more.

[xArtx]

I was wondering if it is at all compressed?

ie. Do (older OCS) Amigas do the frequency range a modern hifi can reproduce 20-20,000 Hz or near?

[mc6809e]

The key to understanding Amiga audio is to realize that Paula is really a synthesizer chip and not just a sample player. Arbitrary waveforms can be output at arbitrary rates.

Standard PC audio is usually a PCM stream which is output at a fixed rate.

[john1979]

Originally Posted by xArtx

I was wondering if it is at all compressed?

ie. Do (older OCS) Amigas do the frequency range a modern hifi can reproduce 20-20,000 Hz or near?

Originally Posted by TheInternet

Models released before Amiga 1200 also have a static "tone knob" type lowpass filter that is enabled regardless of the optional "LED filter". This filter is a 6 dB/oct lowpass filter with cutoff frequency at 5 kHz (some circuit diagrams indicate cutoff frequency at 4.5 kHz, though)

So for older Amigas the answer is no, sound above 4.5khz is cut off, unlike the "power light filter" this can't be disabled.

Amiga 1200 and 4000 doesn't have a lowpass cutoff at 26khz I believe, so with AHI you can have 14bit Hi-Fi quality sound very close to today's modern soundcards. No idea what the low frequency response, SnR and distortion levels are like though. Probably not as good as a decent soundcard.

I spent ages wondering why my A600 sounded so dull compared to my 1200. I even had the capacitors replaced. Then I found out about the cutoff. Doh!

Edit:
Also see this link https://bel.fi/alankila/modguide/interpolate.txt
and this one: http://www.amiga.org/forums/showthread.php?t=61133

As for emulators I don't think WinUAE or any of the PC MOD players sounds exactly a real Amiga. I believe the Amiga's DAC works a little differently to a modern DAC and that maybe why (something to do with being non-linear). Someone with deeper knowledge of Paula will have the answer.

[xArtx]

5kHz is still only the higher end of mid-range. I wonder why they added a hardware filter.
Maybe the small amount of analogue circuitry to provide the line out isn't too good with it,
or maybe they assumed smaller speakers in TVs and monitors, and didn't want to send them
the higher frequencies that would only serve to distort those speakers.

I find it interesting that depending on where the low end cuts off,
the entire frequency range could fall within the range of a vintage tube amplifier.
Might have to try that.

[john1979]

I think you're right. The rather basic DAC in the Amiga doesn't do any oversampling, which would reduce aliasing artifacts. So they just bunged the hard filter in there instead. Then on top of that a toggleable filter to reduce high frequencies even further, useful for certain sounds which alias strongly (like the engine revving in stunt car racer)

Thankfully the A1200/4000 doesn't have this restriction and they sound much brighter, and I find the aliasing a welcome part of the Amiga's characteristic sound.

[xArtx]

Maybe there's something in the older Amigas running slower, and the source not coping
well at the high end, I'm a bit out of bounds there without much knowledge of Paula.
If the hardware filter is just a resistor capacitor network,
that could be found out by removing it and using a modern hifi.

Don't know how it translates to UAE if it's still a Paula emulation trying to do the same
thing, at the same slow clock speed.

I do have a 1962 stereophonic valve amp that has been modernised with digital optical input.
Typically, the audio valves in the amp compress the audio somewhat,
and the audio output transformers typically aren't even capable of delivering the
very high or very low end by today's standards.

So you can have come pristine audio from a class A amp on the cheap,
but have to be selective about the source music, because it can still be noticed
if it sharply cuts some high frequencies... which varies track to track,
and it's now a "shelf unit" with midrange drivers only,
but some analogue processing to push the spectrum into those driver's range.

[TCD]

http://en.wikipedia.org/wiki/Original_Chip_Set#Audio

[mc6809e]

Originally Posted by john1979

So for older Amigas the answer is no, sound above 4.5khz is cut off,

Not quite.

The fixed low pass filter's cutoff isn't very steep at all so plenty of energy above 5KHz is still passed.

I think even at a frequency of 18KHz about 20% of the signal still gets through. The attenuation with increasing frequency is very gradual.

[pandy71]

Each Amiga have mandatory integrator filter to remove PWM modulation from AUDxVOL functionality (this functionality can be switch off but integrator made on first OPAMP remain).
So ANY Amiga anyway cuts everything before 56KHz (first order so mild slope).

[john1979]

Originally Posted by pandy71

So ANY Amiga anyway cuts everything before 56KHz (first order so mild slope).

Just wondering but why did the hardware designers even consider cutting this frequency sloping downwards?

Any noise above 21khz would have been inaudible.

Most of us would struggle to hear anything above 20khz, even when in our teens.

[Loedown]

Originally Posted by john1979

Just wondering but why did the hardware designers even consider cutting this frequency sloping downwards?

Any noise above 21khz would have been inaudible.

Most of us would struggle to hear anything above 20khz, even when in our teens.

Indeed you are right but consider the record versus CD argument, from what I understand of records they can reproduce sounds from DC - 100 KHz and although we cannot hear those higher frequencies they still interfere with the audible frequencies which we can hear ( harmonic addition / subtraction ). The idea of waves on a pond from two separate pebbles. I can also confirm that Kraftwerk on vinyl sounds better than Kraftwerk on CD. I've even done a side by side comparison of first pressing vinyl versus remastered CD.

[Loedown]

The grittiness of the Amiga's 8 bit audio is something highly desired by today's standards and there's many programs that will allow aliasing and alike. This rather odd find on Youtube highlights where this is desirable. Think Aphex Twin / Squarepusher / Mu-ziq

[ Show youtube player ]

[Thorham]

Originally Posted by Loedown

The grittiness of the Amiga's 8 bit audio

I keep reading this, but is it really true? Isn't it just the samples that are used? When I play CD quality WAVs in 44khz 14bit on my A1200 (with calibration, without AHI), it sounds almost as good as on the peecee, and there certainly isn't any audible grittiness.

[mc6809e]

Originally Posted by Thorham

I keep reading this, but is it really true? Isn't it just the samples that are used?

ARG!

This idea that the Amiga merely plays samples has to die!

The Amiga outputs arbitrary waveforms at arbitrary rates, somewhat filtered. Sometimes those waveforms come from samples of recorded sound and sometimes they are completely synthetic.

A two byte synthetic waveform of -128, 127, for example, will produce a somewhat filtered square wave. If a period value of 7159 is used, that square wave when output will have a fundamental frequency of 250 Hz. But it will also include many odd harmonics, too, like 750 Hz, and 1250 Hz and so on, well past 5KHz as the output jumps from -128 to 127 and from 127 back to -128.

The "grittiness" of the Amiga's sound comes from all these additional harmonics that are produced as the output jumps from one value to another. The more slowly the waveform is output, the more additional harmonics appear in the output EVEN IF THEY WEREN'T PRESENT IN THE ORIGINAL SAMPLE.

[Thorham]

Originally Posted by mc6809e

The Amiga outputs arbitrary waveforms at arbitrary rates, somewhat filtered. Sometimes those waveforms come from samples of recorded sound and sometimes they are completely synthetic.

They always come from samples, because samples are all the Paula chip can play. It's not a synthesizer. Sure, you have arbitrary sample rates, and you can modulate the volume of one channel with another, but it's still sample based.

[john1979]

Originally Posted by mc6809e

ARG!

This idea that the Amiga merely plays samples has to die!

The Amiga's synthesis abilities are extremely limited. Paula is nothing like a synth. For the most part it's a dumb sample player. Of course there is software synthesis but that is not the same.

There is some ability of limited FM synthesis. But I'm not aware of it ever getting much use. If any?

@Loedown
Yes, I had forgotten about harmonics. Thanks

[Loedown]

Originally Posted by Thorham

I keep reading this, but is it really true? Isn't it just the samples that are used? When I play CD quality WAVs in 44khz 14bit on my A1200 (with calibration, without AHI), it sounds almost as good as on the peecee, and there certainly isn't any audible grittiness.

8 bits range is 256 ( 0 - 255 ) audio levels. At 5 volts this is steps of ~ 0.02

14 bits range is 16384 ( 0 - 16383 ) audio levels. At 5 volts this is steps of ~ 0.00030

For a better comparison save two jpeg images from the same original file only do one at high grade and one at low grade and you will see the difference, audio is no different to pictures in this respect.


Nyquist's fundamental law is that to sample a frequency you need to double the speed of the frequency of the sound you are trying to sample and that's why CD does 44.1 KHz ( 22.05 KHz max frequency to sample ) which is of course enough for our hearable audio spectrum but refer back to my comment about LP Vs CD where LP and the system you are using, can go as high as 100 KHz.

Also regarding Amiga samples, it depends on the output frequency that the sound is being played at and the speed that the original sample was recorded at, if both are almost divisible by one another you will have very little aliasing at all but if they are tremendously different then you will hear much aliasing. String sounds are the best for hearing of horrible aliasing because the original sounds usually use phasing between two oscillators to achieve a richer sound ( slight pitch phasing ).

[Loedown]

Originally Posted by john1979

The Amiga's synthesis abilities are extremely limited. Paula is nothing like a synth. For the most part it's a dumb sample player. Of course there is software synthesis but that is not the same.

There is some ability of limited FM synthesis. But I'm not aware of it ever getting much use. If any?

@Loedown
Yes, I had forgotten about harmonics. Thanks

Paula only does samples. It can do AM and FM modulation forms and that is done in hardware. Information taken from Amiga Hardware Reference Manual.