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Old 12 March 2017, 23:04   #1
Foebane
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Audio Filter "Emulated (A500/A1200)" - What's the difference?

Can either A500 or A1200 version be used interchangeably?
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Old 13 March 2017, 11:04   #2
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Let me elaborate:

A while ago, I created a configuration for an A1200 spec to use HDDs, for WHDLoad. Naturally, it had the A1200 audio filter, but then I took that same configuration and saved it again under "Host" so I could use it for the host configuration on Quickstart. Now I find I'm using an A500 or A600 (selected from Quickstart) and the A1200 audio filter is being used on it.

1. I've done a listening check of both and I don't really hear any difference. Is there really any? Would it be better to use the A1200 filter on A500 or A600?
2. How does the host configuration load on Quickstart? It seems that selecting an Amiga model only changes the Hardware settings, although it seems that some Host settings (like filter) are changed too. Is that right?
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Old 13 March 2017, 11:09   #3
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From what I remember if you enable it, the sound is more like you cut the high frequencies a bit. But maybe I am wrong.
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Old 13 March 2017, 11:44   #4
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From what I remember if you enable it, the sound is more like you cut the high frequencies a bit. But maybe I am wrong.
I know, but what's the difference between the A500 and A1200 filters? Or is there any? There must be a reason for the two separate options in "audio filter emulated".
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Old 13 March 2017, 11:53   #5
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Quote:
Originally Posted by Foebane View Post
I know, but what's the difference between the A500 and A1200 filters? Or is there any? There must be a reason for the two separate options in "audio filter emulated".
A500 has an extra low pass filter in the audio path compared to the A1200.
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Old 13 March 2017, 11:55   #6
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Originally Posted by demolition View Post
A500 has an extra low pass filter in the audio path compared to the A1200.
So the A500 actually has two filters compared to the A1200's one?
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Old 13 March 2017, 11:56   #7
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Yes, and one of the two is permanently on.
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Old 13 March 2017, 12:15   #8
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So is it better to use the A500 with the A1200 audio filter, even though it's not authentic? It seems I don't have much choice, anyway, because my Host configuration sets it that way. Besides, I wouldn't want to use an A1200 with the A500 audio filter, so I guess saving the Host configuration with the A1200 audio filter is a better bet.
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Old 13 March 2017, 13:11   #9
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One is not objectively better than the other - it depends on which one you prefer. Most people prefer the A1200 sound though.
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Old 13 March 2017, 14:29   #10
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Most people prefer the A1200 sound though.
Yeah, I do, too.

As an experiment, I tried removing the two "filter" lines in the Host .uae file, and both times when I saved it, they returned, calling the A500 filter "standard" and the A1200 filter "enhanced". So if even the developers of WinUAE think the A1200 filter is better, that's fine with me!
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Old 13 March 2017, 16:30   #11
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Well the A1200 was a later model, so for that reason you could define it as 'enhanced', although I think it is probably more likely due to cost savings.
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Old 13 March 2017, 16:45   #12
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The A600 also has two filters I believe, and personally I have modified all of mine by removing the fixed ones. It achieves the "cleaner" sound of the A1200.
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Old 13 March 2017, 17:33   #13
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Why did Commodore even PUT filters into the Amiga machines to begin with? I doubt it was an Amiga decision.

What I know is that the filter being on or off was indicated by the power LED being dim or bright (which was which, I can't remember) and everyone remembers what an Amiga game or demo sounded like when it crashed, when the LED switched between dim and bright just prior to the famous guru meditation, and the audio would change likewise.
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Old 13 March 2017, 18:14   #14
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Why did Commodore even PUT filters into the Amiga machines to begin with? I doubt it was an Amiga decision.
Because the DAC generated overtones which could be attenuated with the low pass filter. The audio was probably more accurate with the filter on (i.e. if you try to play a pure sine tone, it is cleaner with the filter on), but people got used to and liked the sound of these artificial harmonics so in the end most people turned off the filter. The overtones is a part of what makes an Amiga sound like an Amiga, just like the distortion in a SID chip is a big part of the C64 sound.

The filter also limited the frequency range, but as the overtones overlap with part of the audio, they cannot be separated.
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Old 13 March 2017, 18:25   #15
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Amiga sound is perfect, I never hear any "overtones".

At best, there is that irritating distortion you get with some lower Khz samples, but that was down to memory constraints.
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Old 13 March 2017, 18:29   #16
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Amiga sound is perfect, I never hear any "overtones".
From a pure SNR and THD+n point of view, it is very far from perfect. That you do not hear the overtones, does not mean that they are not present.
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Old 13 March 2017, 18:44   #17
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From a pure SNR and THD+n point of view, it is very far from perfect. That you do not hear the overtones, does not mean that they are not present.
Are you sure it's not down to the samples used? I always thought Paula was a simple 4-channel 8-bit DAC with independent volume controls, so there should be no problems, unless the frequency response is uneven?

EDIT: Well, not so simple as that, but I still like the sound of Amiga, distortions and all.
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Old 13 March 2017, 19:11   #18
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After a year of having my A500, I got this item for Christmas 1991:
http://www.bigbookofamigahardware.co...t.aspx?id=1284

I must've thought there was something bad about the audio, probably the filter that I couldn't turn off, so I got the above item, only for it, IMHO, to have too much treble in the audio at the end.
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Old 13 March 2017, 20:57   #19
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It seems my issue is a non-issue after all:

The Host configuration is loaded when WinUAE is launched, so in my case that means A1200 filter, regardless of what Quickstart model is selected. The same happens if I click on the Host configuration drop-down list.

However, selecting a different Amiga model on Quickstart overrides the Host configuration in favour of the selected model, unless the above in the previous paragraph happens.

But I've noticed a bug with selecting the A1000 model, in that the filter will be "Always on (A500)" and stay "always on" with every subsequent model you select, whether it be A500 or A1200. Why is that bug persisting?
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Old 13 March 2017, 21:40   #20
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Quote:
Originally Posted by Foebane View Post
Are you sure it's not down to the samples used? I always thought Paula was a simple 4-channel 8-bit DAC with independent volume controls, so there should be no problems, unless the frequency response is uneven?
The low pass filter can be considered a part of the DAC circuit, also known as a reconstruction filter:
https://en.wikipedia.org/wiki/Reconstruction_filter

Mirror images of the frequencies in the audio signal are created around the half of the sampling frequency of the DAC and some kind of low pass filter must be used if you want to remove these, or at least attenuate them.

This theory also applies to the samples being played back which are played at a lower sampling rate than the DAC/Paula itself, thus some kind of sample rate conversion is involved (and nearest neighbor is a very crude way of doing it). The overtones from the sample rate conversion is probably the main reason why you would want a low pass filter, however ideally the filter should be applied separately to each channel with a cut off frequency tracking each sample's sampling frequency, so having just one filter and being able to turn it on and off was probably as good as what could realistically be done.
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