Sound Blaster Pro Low / High Filter question

[Mau1wurf1977]

It's been pretty quiet on QuestStudios lately...

Anyway I'm investigating something I read on the Wikipedia article of the Sound Blaster Pro. It mentions

a crude high pass or low pass filter

.

So I had a look in the mixer of my Sound Blaster Pro 2.0 OEM and found this:



Oddly it's under "ADC Input select" so I'm not sure where this selection applies. Or maybe it applies only to the digital portion?

I made a quick recording with each filter setting of DOOM and uploaded the results:

http://www.mediafire.com/?lbf8sjemkk6demm

So yea if someone can shed some light on this topic...

[Great Hierophant]

That is the input filter you are referring to, which only deals with sounds coming into the card from a microphone, line-level source, or CD.  Therefore, it would not apply to sound output from a program using the card.

[Mau1wurf1977]

That is the input filter you are referring to, which only deals with sounds coming into the card from a microphone, line-level source, or CD.  Therefore, it would not apply to sound output from a program using the card.

Ok I found another area where there is an output filter switch. Now I planned on using the SQ4 intro for recording and noticed that if you quit the game and run it again, the volume is much lower. Is it possible that Sierra resets the mixer before quitting the game?

I'll upload the two recordings shortly (one with filter ON and one with filter OFF).

[Mau1wurf1977]

Ok so I used the tool SBP-SET.EXE in the SBPRO folder and it game me options for setting the mixer and one option seems to turn the output filter ON or OFF:

SBP-SET.EXE command options



Boot message confirming the mixer and filter settings!





So I did a recording of the SQ4 intro because it has FM and speech. However I struggle to detect a difference :-( If you guys can hear anything of interest please let me know! I'm also wondering what the default setting for the filter is. I take it it's ON?

Here are the recordings in a convenient ZIP archive: http://www.mediafire.com/?11tpxwf9wahzsdc

[Mau1wurf1977]

Monkey Island 2 recordings: http://www.mediafire.com/?f8lnevabxdeqyzz

[Cloudschatze]

The Sound Blaster programming guide describes the SBPro filter as...

Input - selectable between 3.2kHz (default) / 8.8kHz / OFF
Output - selectable between 3.2kHz (default) / OFF

It isn't stated, but the filter doesn't appear to apply to the FM output (given the limited testing that I've done).


In any event, here are some visual/aural examples that may better demonstrate the filter's effect on PCM output:


3.2 kHz filter (default)



Filter off

[Mau1wurf1977]

Cool thanks.

So it this what is called a low pass filter?

Now in layman's terms, what does it do and is it recommended to leave it ON? And why would you turn it OFF?

I simply couldn't detect a difference in the SQ4 Intro recordings (they include digital speech). To be honest I epxected a noticable difference...

[Cloudschatze]

Well, here goes nothing...

The lowpass filter basically allows the lower frequencies to "pass through," while rolling-off those above the cutoff frequency (3.2 kHz, in this case) - not unlike what can be accomplished with equalization, or a treble cut.

This filter helps reduce the audible aliasing/artifacts/hiss in lesser-quality recordings, and while it can be beneficial for most game audio samples (and is one of the reasons I'm sort-of partial to the SBPro(2), having it on for anything over 22050 Hz results in a noticeable and unnecessary loss of fidelity (as the recordings I posted above demonstrate).

I haven't checked to see what rate is used for the SQ4 samples. I imagine it's pretty low, which would explain the filter's lesser effect.


* (Ken Pohlmann's "Principles of Digital Audio" has a detailed, technical description, if ever you're interested.)

[Mau1wurf1977]

So I made another recording and now we are getting somewhere!

The Game is Indiana Jones and the Fate of Atlantis. This time I muted the FM synth in the mixer. I recorded DOSBox as a reference so to speak, AWE64 GOLD and SB Pro 2.0 with output filter ON and OFF.

Now the difference between Filter ON and OFF is quite amazing. The AWE64 and SB Pro 2.0 with filter ON sound somewhat "limited" or even "muffled". The DOSBox recording is outstanding and the SB Pro 2.0 recording with filter OFF is very close to the DOSBox recording.

It's very bright and you can hear a lot more highs. Quite fascinating and I would definitely say I prefer the SB Pro 2.0 with filter OFF for speech over the AWE64 GOLD. I haven't tested the SPDIF yet however...

I will upload all the files shortly!

Here are the 4 recordings:

- SB Pro 2.0 with Filter ON (default)
- SB Pro 2.0 with Filter OFF
- AWE 64 GOLD (note that this card has bass and treble controls, but I left it at default which is 2 steps above the middle setting)
- DOSBox 0.74

http://www.mediafire.com/?lhapzisap9rnll7

[endre1952]

The Amiga computer, which used rather low-quality 8-bit samples, had a low-pass filter that could be turned on and off by software. It was easy to tell the difference.

[Mau1wurf1977]

The Amiga computer, which used rather low-quality 8-bit samples, had a low-pass filter that could be turned on and off by software. It was easy to tell the difference.

That's interesting, I didn't know that!

What did you think of the "Fate of Atlantis" recordings? What's your take?

[Great Hierophant]

The Amiga computer, which used rather low-quality 8-bit samples, had a low-pass filter that could be turned on and off by software. It was easy to tell the difference.

That's interesting, I didn't know that!

What did you think of the "Fate of Atlantis" recordings? What's your take?

The Sound Blaster Programming Manual advises to turn off the low-pass filter for stereo recordings or those recordings using high sampling rates.  I assume the view is at 22050kHz or above, the sampling rate should be generally sufficient to capture the sound without the high-frequency aliasing that occurs at lower sampling rates. 

When the Pro is first initialized, is the filter on?  And assuming no user intervention, when a game like FOA is started, does the speech sound like to the filtered recording?

[Mau1wurf1977]

When the Pro is first initialized, is the filter on?  And assuming no user intervention, when a game like FOA is started, does the speech sound like to the filtered recording?

Good question! I will report back...

The AWE64 GOLD recording was all default though. So it seems to have a filter on by default. I couldn't find an option to disable it.

Update: Yes the output filter is ON by default on my Sound Blaster Pro 2.0

[endre1952]

Definitely, the AWE64 and filtered SBPro 2.0 recordings have a more muffled sound to my ears. It's similar to what is heard on the Amiga with the low-pass filter turned on.

[Mau1wurf1977]

Definitely, the AWE64 and filtered SBPro 2.0 recordings have a more muffled sound to my ears. It's similar to what is heard on the Amiga with the low-pass filter turned on.

Yea "muffles" describes it quite well. So far I haven't found a way to disable the filter on the AWE64. I will play with the SPDIF out shortly and hoping this works well...

[Mau1wurf1977]

So I used the "Frequency Analysis" tool in Audacity and ran it over my two "Fate of Atlantis" recordings:

Frequency Analysis of "Fate of Atlantis" recording on Sound Blaster Pro 2.0 with Output filter ON



Frequency Analysis of "Fate of Atlantis" recording on Sound Blaster Pro 2.0 with Output filter OFF