What IS audio on a PC anyway?

re: What IS audio on a PC anyway?

Simon Cooke [exMSFT] — Tue, 26 Oct 2004 20:34:00 GMT

A couple of quick notes...

I was under the impression that phones used 8khz 8 bit data, which would be closer to 7.8kb/s, not 11kb /s.

Also, the "pops" and" brain smearing" arguments are not comet at all; there is hardware filtering after the DAC which bandwidth limits the signal and forces it to be a smooth continuous waveform. The speaker care itself also has a mass, and the inertia and momentum of the speaker itself also has a very similar effect on the signal, causing it to roll off the higher frequencies.

So basically, no pops. Sure, you can create pop 'sounds' but only at the limiting frequency cf the output of the DAC. The sound of the transformer really is a 60 hz sound - the transformer coils resonate at the AC line frequency + distortion - no brain-driven signal integration required.

Simon

re: What IS audio on a PC anyway?

Simon Cooke [exMSFT] — Tue, 26 Oct 2004 20:34:00 GMT

comet = correct. Damn TabletPC!

re: What IS audio on a PC anyway?

Larry Osterman — Tue, 26 Oct 2004 20:37:00 GMT

Simon, you're right, the analog filtering after the DAC smears them out, but from a conceptual standpoint, that was the easiest way of explaining the idea that these are discrete samples.

And 60Hz is above the human threshold of hearing, which is why you can hear the transformer, I chose it because it was the most common example of a low frequency sound I could come up with.

re: What IS audio on a PC anyway?

Larry Osterman — Tue, 26 Oct 2004 20:38:00 GMT

Oh, and you're right, phone is 8kHz, not 11kHz. That's why I indicated that it was for devices <i> like </i> telephones.

CD bandwidth calculation is wrong

Todd — Tue, 26 Oct 2004 20:40:00 GMT

Your units are wrong on your math. Your "44,1000" also has one too many 0s. The correct formula is:

44,100 (1/s) * 16 (b) * 2 = 1411200 b/s = 1378.125 Kb/s = 1.35 Mb/s.

That's in bits. In bytes, you get:

(44,100 (1/s) * 16 (b) * 2) * 1/8 (B/b) = 176400 B/s = 172.27 KB/s = 0.17 MB/s

Thus, your numbers are correct but your units (bytes vs. bits) are wrong assuming the standard notataion of 'b' = bits and 'B' = bytes. I also used 1024 B/KB (b/Kb, KB/MB, Kb/Mb) rather than 1000.

re: What IS audio on a PC anyway?

Simon Cooke [exMSFT] — Tue, 26 Oct 2004 20:41:00 GMT

Strangely, given the topic, this is one case where a picture really is worth a thousand words.

re: What IS audio on a PC anyway?

Simon Cooke [exMSFT] — Tue, 26 Oct 2004 20:43:00 GMT

oh, and re: the telephone stiff, I was referring to this line, Larry.

"The bandwidth of a telephone conversation is 88kB/second, or 11kb/second, an order of magnitude lower."

re: What IS audio on a PC anyway?

Larry Osterman — Tue, 26 Oct 2004 20:45:00 GMT

I'd like to do a picture, but I'm not good enough in Mathematica to do justice, and I'm not going to steal someone elses work.

re: What IS audio on a PC anyway?

Jeff Parker — Tue, 26 Oct 2004 20:46:00 GMT

Now another question along the same lines, in like Media Player, the visualizations when listening to music, like the graphical equalizer and such. Do those pretty much tap into the same signal to draw on the screen? I would assume so, but not sure, one of those things I just sat back and enjoyed was curious but never dug into it.

But I am assuming those pulses for the speaker are the same pulses that you see on the screen.

re: What IS audio on a PC anyway?

Larry Osterman — Tue, 26 Oct 2004 20:47:00 GMT

Yup, visualizations operate on the samples being sent to the sound card. Internally they're implemented as dshow filters that render their samples to the screen instead of performing some kind of transform on them.

re: What IS audio on a PC anyway?

Audio dude — Tue, 26 Oct 2004 21:04:00 GMT

Might want to clarify that you're talking about cellphones, not landline phones. The yunguns might be confused.

You might also clarify that you can transfer PCM over S/PDIF, but that isn't the only data format available. Further, in the PCM case, the PCM data might be encoded for SPDIF on the card itself, not on the host PC. :)

re: What IS audio on a PC anyway?

Simon Cooke [exMSFT] — Tue, 26 Oct 2004 21:25:00 GMT

Larry - send me an email, and let me know what you need. I should be able to knock something out pretty quickly.

re: What IS audio on a PC anyway?

Eric Lippert — Tue, 26 Oct 2004 21:31:00 GMT

> One thing to realize about PCM audio is that it’s extraordinarily sparse – there is a huge amount of compression that can be done

Well, sure -- but it is important to note that in many compression schemes, it's lossy compression.

Compression works well when there are large ranges with small variations and patterns in the data, both of which are true of SOME audio. Compression works great on "bassy" music because a 220 Hz note has a profile that looks like a sine wave with two hundred samples per wavelength.

There are therefore two ways you could compress this. You could take advantage of the small change between any two samples and just store the differences, which take up fewer bits than the values. Or, you could take advantage of the fact that a 220 Hz sine wave is computable if you know its duration, initial value, and frequency -- just store them!

Lossy compression schemes do exactly that -- they take the Fourier transform of the signal to determine what combination of sine waves make up a particular sample, and then just store information about those sine waves.

The question then is "what about the high-frequency stuff?" The human threshhold for hearing is ~20KHz. Any signal sampled at 40KHz that contains loud sounds in the KHz range is going to have large variations between samples, and the details of the overall wave shape are likely to change rapidly.

That's what makes lossy compression schemes lossy. They just throw away the high frequency information because its too hard to compress!

That's why when you listen to overly compressed audio, things like applause and symbol crashes sound awful. Applause and symbol crashes tend to have lots of random, hard-to-compress, high amplitude, high frequency signal in them.

re: What IS audio on a PC anyway?

Ziv Caspi — Tue, 26 Oct 2004 21:49:00 GMT

Actually, modern audio compression algorithms can do much more than simply cut off the higher frequencies -- encoders like MP3 have a psycho-acoustic model of the way we hear things to improve compression even further. For example, a strong tone will often "mask" a weak tone which is close to it in frequency, so the second one can often be thrown out as part of the lossy compression.

re: What IS audio on a PC anyway?

Chris — Tue, 26 Oct 2004 21:59:00 GMT

I've read that a sample rate higher than 2x the highest sampled frequency is unnecessary according to Nyquist's theorem. If this is so, and human hearing tops out at around 20KHz (or let's say 30KHz for the super humans), then sample rates higher than 40-60KHz are just taking up more space on our storage media for no real gain.

Some say that there are still effects from those inaudible frequencies on those in the audible range. If that is so then recording microphones (which also top out at around 20KHz) would still record the effects of those frequencies. Any other improvement in perceived sound quality is attributed to the quality of the equipment in the recording and/or playback chain (or is psychological).

Search for Dan Lavry on usenet or elsewhere for a more in depth discussion/argument of this.

re: What IS audio on a PC anyway?

Simon Cooke [exMSFT] — Tue, 26 Oct 2004 23:01:00 GMT

Chris - there is a subtlety here.

The Nyquist limit details how many samples one must take to accurately reproduce a signal of a given frequency without aliasing.

If a sound was generated by a single staionary point source in an infinitely absorbing room (ie. no echo), then Nyquist will tell you everything you need to know to reproduce that sound.

However, when you start positioning sounds in space, higher frequencies become important. While the human ear can only hear frequencies up to about 22.5kHz (on average - some people can hear more, some less), it can discriminate between the arrival times of sounds at much higher resolution - on the order of what would be a frequency of 100,000Hz. That is, if the same sound wave arrives 10 microseconds apart, at one ear first and then the other, the listener can tell the difference, and interprets this as spatial separation of the sounds.

A lot of positioning information is encoded in the higher frequency domain. So while Nyquist is strictly correct for a given signal, it's a very much idealised form when you're dealing with stereo positional audio.

re: What IS audio on a PC anyway?

Andreas Wölfer — Tue, 26 Oct 2004 23:40:00 GMT

Wow.

Thanks for that.

re: What IS audio on a PC anyway?

Norman Diamond — Wed, 27 Oct 2004 02:01:00 GMT

10/26/2004 2:31 PM Eric Lippert

> That's why when you listen to overly
> compressed audio, things like applause
> and symbol crashes sound awful.

Ann if ewe overtly comprise dictionaries four spilling chequers, sings like cymbal clashes look awe full? @ leased they sound like symbol crashes.

re: What IS audio on a PC anyway?

Paul Winwood — Wed, 27 Oct 2004 08:41:00 GMT

The other factor is the dynamic range (loudness) of human hearing. We can hear over a range of 120dB although sustained levels of over 90dB can damage our hearing. CD audio can achieve about 96dB. AC-3 and DVD-Audio formats achieve more than this.

re: What IS audio on a PC anyway?

Petr Kadlec — Wed, 27 Oct 2004 12:25:00 GMT

Simon: "about 22.5kHz (on average - some people can hear more, some less)"

Are you sure about that? I've always thought that it is significantly lower (somewhere around 16 kHz). In fact, if this would be true, it would mean that some people would be able to actually hear that there is missing some high frequency signal on a CD audio.

re: What IS audio on a PC anyway?

Stefan Kuhr — Wed, 27 Oct 2004 13:50:00 GMT

>>"there's a high pass filter that smooths the output of the signal."

Just one comment: It's not a high pass, it is a *low* pass filter that is used to suppress the frequency portions that are "mirrored" into the signal from the next band (overlaid by a sinx/x curve), assuming it is not an ideally bandlimited signal that you are sampling. The low pass should let pass all frequency portions from DC to the highest frequency (i.e. 20kHz) and should then have a very steep curve to suppress everything at or above half the sample frequency (i.e. 22.1kHz). This is also where the quality of the analog circuitry comes into play: You can create very steeply curved low pass filters with only few RC elements (e.g. Chebychev filters) but those have a non constant group delay (some people can actually hear this), which means that e.g. the lower frequencies arrive later at the listener's ear than the higher frequencies or the other way round. If you want steep filters but constant group delay you need more RC elements in the analog filter and thus more complex and expensive analog filters (e.g. Bessel filters).

re: What IS audio on a PC anyway?

Darren Cherneski — Wed, 27 Oct 2004 14:44:00 GMT

When I was taking engineering at university one of my profs mentioned that transformers hum at twice the input voltage. So transformers in North America actually hum at 120Hz, not 60Hz.

Here's a link that explains why:
http://www.federalpacific.com/university/transnoise/chapter3.html

re: What IS audio on a PC anyway?

Roland Boon — Wed, 27 Oct 2004 17:35:00 GMT

There is a different method of raw audio encoding than PCM which is used by Super Audio CD; a 1 bit digital stream known as DSD.

"The DSD technology uses a sampling frequency of 2.8224 MHz, which is 64 times higher than that of CD. This enables a frequency response up to 100 kHz and a dynamic range of 120 dB across the entire audible range."

From http://www.licensing.philips.com/information/sacd/protec/documents1089.html

re: What IS audio on a PC anyway?

Simon Cooke [exMSFT] — Wed, 27 Oct 2004 17:48:00 GMT

Petr - sorry, I meant 22.05kHz :) I missed a decimal.

Nyquist states that you sample at twice the frequency of the signal you want to reproduce (with the caveats I previously mentioned re: spatial positioning).

CD audio was chosen to record at 44.1kHz because most people top-out their hearing at the high end at 22.05kHz (which, by Nyquist, is sampled at 44.1kHz).

DAT tapes record at 48kHz because they didn't want them to be compatible with CD audio on a direct binary level - the idea being to reduce piracy. In actuality, all that really happened because of it was tape went the way of the dodo.

re: What IS audio on a PC anyway?

John Topley — Wed, 27 Oct 2004 19:42:00 GMT

DAT is still used in the music industry though.

re: What IS audio on a PC anyway?

Jeremy Brayton — Wed, 27 Oct 2004 19:51:00 GMT

Times like this make me wish I had your brain. I don't really want to go work for the Windows Audio team, but it's almost as if I don't really have to if I could just get exclusive access to just SOME of your brain. If there was just some way of harnessing it safely and cheaply.

I guess I'll just have to settle for more posts like this. I used to think some of this stuff was over my head, but it really isn't if it's given in such a clear explaination. One can get lost in the techno-babble of the audio world quite easily, but somehow I understood everything that was said.

Thanks again.

So how do I play sounds anyway?

Larry Osterman's WebLog — Wed, 27 Oct 2004 23:03:00 GMT

re: What IS audio on a PC anyway?

Stewart — Thu, 28 Oct 2004 07:15:00 GMT

I remember being told that one of the other problems with the sampling rates is that before encoding the source signel must be low pass filtered to prevent frequencies above the nyquist limit sneaking through and causing aliasing.

Because this filtering cannot be ideal (because of the group delays mentioned above) you lose some of the higher frequencies below the nyquist limit. If you expand the sample rate, you can make the filter higher and you save more of the perceptible audio

re: What IS audio on a PC anyway?

ATZ Man — Thu, 28 Oct 2004 17:59:00 GMT

Interesting article and comments.

48KHz was already established as a standard for digital audio when CD Audio was being defined. Steward seems to state the motive for 48KHz. The 44.1KHz standard supposedly comes from making the physical CD fit in a Japanese-size car stereo slot and still hold a certain amount of music.

I remember in the late 80's PC games were attempting to use the original PC speaker to output digitized sound. It was worse than phone audio, and compute intensive, and 24-bit audio cards in a retail box cost $30 so it's not much use.

re: What IS audio on a PC anyway?

Norman Diamond — Mon, 01 Nov 2004 00:02:00 GMT

10/27/2004 5:25 AM Petr Kadlec

>> Simon: "about 22.5kHz (on average - some
>> people can hear more, some less)"

[or 22.05 kHz, it doesn't matter much to this]

> I've always thought that it is significantly
> lower (somewhere around 16 kHz).

The average varies with age and gender. For adult men it might wee be 16 kHz. But the overall average is still the overall average.

> In fact, if this would be true, it would
> mean that some people would be able to
> actually hear that there is missing some
> high frequency signal on a CD audio.

That's exactly true. No matter what the average is, that means some people are below it and some people are above it. There always have been people who are put off by the missing high frequencies on CD audio -- and by inconsistent phase shifts, and on white noise or pink noise added by ordinary electronic amplifiers.

10/28/2004 10:59 AM ATZ Man

> The 44.1KHz standard supposedly comes from
> making the physical CD fit in a Japanese-
> size car stereo slot and still hold a
> certain amount of music.

Huh? Japanese car stereo slots are the same size as import car stereo slots. Of course there aren't a lot of imports, and most of those are from Germany not from the NAFTA zone, but they all take the same kinds of stereos.

There are other kinds of optical disks such as MDs, but those didn't exist when CDs were first developed, and they sure didn't figure in setting frequencies. I've read that MDs use a lossy compression algorithm. They were popular for a while because of their portability (e.g. can be worn while riding a train) but that market has moved to flash memory.

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