Open Music Labs Forums

Hey everyone!

I've been watching this thread with interest as well as Jeri Ellsworth's video.

I decided I'd like to experiment with using the Commodore 1541 5 1/4" for truly floppy audio. To start with, I have disassembled two of them, and gotten rid of the useless, huge, transformer-based power supplies and the motherboard.

Next, I'm taking an information theoretic approach, trying to estimate record time, bit rate, etc.

The original motor speed is approximately 300 RPM. That means 5 revs/second or 200 milliseconds per revolution. There are 35 tracks, so total record time is approximately 7 seconds. Not bad! That's as long as a microdec, no? :)

Anyway, my friend and I were wondering about audio quality. From what I've read, the drive has 17 to 21 sectors per track. Each sector is 256 bytes. This means each track has between (17*256*8=34816) and (21*256*8=43008) bits on it. So, let's work with an average of 38kbits, b/c this is floppy audio, not a mars rover.

So one thing we may want to compute is the bit rate of audio coming off of the disk. It looks like it can stream data about 38k/.2 = 190kbps. Pity the good old C64 couldn't get anywhere that fast!

To estimate audio quality, I would like to plug this number into the Shannon-Hartley theorem which states that C = B*log2(1+S/N), where C = bitrate and B is bandwidth and S/N = linear signal to noise ratio. Note that by comparison, a CD player has a bitrate of approximately 750 kbps per channel, about 4 times better than a 1541!

What is the bandwidth, given: C = 190kpbs and desired S/N > 8-bit, e.g. 256:1? Well B = C/(log2(1+S/N), which is 190k/(~8) ~= 23.750 kHz! That's not bad!

Of course, that's only at 8-bit quality! What if we want, e.g. 12-bit quality? Then B = 190000/(~12) = 16kHz! That's not CD quality, but it's still the highest frequency many can hear! Woo-Hoo! This is encouraging.

However, I should add that this is only a *theoretical maximum* information storage limit. These limits are only approachable in the real-world using complex modulation schemes. I don't plan to do any of that! It's floppy audio, not a robot-guided stent.

On the other hand, the original drives had more bits on there for framing purposes, which means the bandwidth of the medium may a bit greater. It looks like they used 5 bits to encode 4, so bandwidth may be up to 25% greater (half a bit!) than base estimation.

Ok, that wraps up tonight's exploration. My drives are 10 miles away in my studio and I'm going to get a good night's rest. Here's the summary of my first exploration of using the Commodore 1541 5 1/4" floppy drive for audio:
Total record time: 7 seconds ( 35 loops of 200 milliseconds each)
Maximum frequency at 12-bit quality: 16kHz

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http://diydsp.com http://diydsp.com ? http://diydsp.com !

Okay! So, yesterday I started considering some of the theoretical math related to audio quality when using a Commodore 1541 for audio. Prior to that, I had disassembled my 1541s and extracted only the drive part - no circuit boards or power supplies, etc.

Here are two photos showing what the insides of a 1541 look like. View these images separately in your browser to see as much detail as possible.

In the top view, you can see four plastic headers with different numbers of wires coming off of various places on the drive. From left to right, these are the 1. Stepper motor header, 2. Read/Write head, 3. LED Header, 4. DC Spindle motor. Note also the sliding drive head and and a small orange single-sided through-hole circuit board and the classic Commodore brown plastic drive window.



In this photo of the underside, notice the spindle motor and the stepper motor. Also, the underside of the read/write head is found in the center of the photo. The read/write read has two small components soldered onto it, a diode and a resistor (15k?).

Ok, I admit that last post was a bit underwhelming.

Well I did that on purpose, but it's about to get fascinating. Here is the read/write head on the 1541:


Here is the header for the read/write head. If you are reading this and haven't read Macanulty's article on read/write heads on this forum, go check it out.



On the 1541, we are lucky because the header terminals are larger than the ones found on new-fangled 3 1/2" Steve Jobs-style diskettes. We are double happy because as you can see, the manufacturer has taken the time to clearly identify which of the signals is ground and which ones are for the heads. Just like Dan's drive, there are three heads. I don't know which is which yet. I measured their impedances and they came out to be: 14, 16.3 and 13.2 Ohms.

I started to wonder if these were maybe supposed to be roughly identical and I'm seeing manufacturing variations. After all, I assume they're supposed to be coils and they're small, so diameter must be rather small leading to variation in cross sectional resistivity? Who cares though, really! We're listening to ska Christmas carols and making *floppy music* here, folks.

Here is a close up of the diode and resistor which form part of the circuitry *right below the head itself*.
Make sure you right-click on this picture and view it in its own tab so you can see it with the benefit of high-resolution. I can see details here that I couldn't see with my bare eyes. Camera FTW.

guest wrote:so what are the limits for analog recording
the floppy audio project reads and writes in analog
so its more a function of noise floor i would think
and tape isnt much better than -60dB

I believe Shannon-Hartley is applicable here. Generally, people use it to compute the bit rate of a noisy channel given the analog bandwidth and s/n ratio of that channel. I've applied it in reverse, saying that if the data rate of the channel was known to be ~240 kbps, then the product of the analog bandwidth and s/n ratio will be equal to *or greater* than that. We can't know how much greater from these calculations until we measure. It depends on the quality of the medium, the head and how much margin the engineers felt comfortable putting into the product :)

Anyway, there is a tradeoff between s/n ratio and maximum frequency. In order to know one, we must know the other, so I tried to find the frequency limits of tape and I haven't found any sources yet that give hard details.

I am dazzled by the various reported values. In this page: http://stephan.win31.de/tech_en.htm a German named Stephan says the dynamic range of cassette ranges from 50-56 dB from Type I to Type IV.

Here's what another guy says about frequency ranges:

A really good cassette deck will have a high frequency response in the range of 16-18 kHz using Type II tape. Metal tape can be as high as 20kHz, and normal bias form 12 - 16, depending on the tape deck and noise reduction used or not used.

Here's another guy:
[quote]
Specs for my ReVox B215 deck:
Type I : 30 Hz 18 kHz +2/-3 dB (Normal)
Type II : 30 Hz 20kHz +2/-3dB (Chrome)
Type IV : 30 Hz 20kHz +2/-3dB (Metal)
[\quote]

yet another estimate:
[quote]
Normal: 14 KHz
Chrome Dioxide: 16.5 KHz
Metal: 19 KHz
[\quote]

So anyway, let's compare the best and wost case possibilities:
Worst tape: Type I, 14 kHz bandwidth, 50 dB dynamic range. Estimated maximum bitrate is 14k*(1+log2(50)) = 93 kbps
Best tape: Type IV, 20kHz bandwidth, 56 dB dynamic range. Est. max birate: 20k*(1+log2(56)) = 136kbps
1541: Calculated bitrate = 5 rots/second * (19 sectors*256 bytes*8 bits =38912) * 5/4 (encoding rate) = 243kbps
CD: Computed bitrate = 44.1 k samples/sec * 16-bit per sample = 705kbps

As calculated in a previous post, given the rotation speed and data density of the 1541, the max bitrate is about 243kbps. So, we can see the 1541 has the *potential* to produce somewhat better quality than a Type IV cassette, but still not very close to CD quality (unless we triple the motor speed?! reducing the record time to 2.3 seconds?)

What this means for your original question about audio quality is that it depends on the bandwidth that we want to record onto the medium. In this model, the medium is noisy enough that at some bandwidth, we simply won't be able to read analog signals from it without a minimum of noise. For example, if we attempt to record full-spectrum information up to 18 kHz, then we compute the linear s/n of what we will be able to read back like this: log2(1+s/n) = 243k / 18k = 13.5 Since s/n is greater than 100, we can safely ignore the "1+" and say that we have a linear dynamic range of about 11586. Therefore, 20*log10(11586) = 81.2 dB @ 18kHz.

I believe the German guy's numbers may be a little pessimistic. There are many more details. For example, I believe the bias signal sucks up some of the bandwidth, too. If it's running at 40kHz, that's approximately double the bandwidth and s/n will go down by 6 dB to 75.2 dB. Another issue: the bandwidth of the cassette tape is not uniformly-spaced across the bands. Anyway, this should give some ballpark information about what to expect.

Let's see... a spinning disk is not exactly the same as tape, but close enough, so let's try it. I mean, it's just floppy audio after all :)

8-bit floppy disk @300 RPM (5 RPS) @maximum radius of 5.25"

Circumference = pi*d = 3.14159*5.25 = 16.5". It's spinning through that diameter 5 times per second, so I believe it's 82.5 ips! That seems rillly fast compared to tape hahaha.

Common cassette: 1.875ips (50-56dB, 15-20kHz bandwidth)
Reel-to-Reel: 7ips gets 9kHz bandwidth, approx. 60 dB
Reel to reel: 15ips gets 18kHz bandwidth
1541 disk, outer diameter: 82.5ips, estimated bandwidth @ 18kHz is 75 - 81.2 dB
1541 disk, inner diameter (est. 2.625"): 41.3 ips.

Oh! now that I've slept a little bit (on my couch, cat peed on the bed) I'm wondering if perhaps the third wire on the motor speed controller is an adjustment that changes the drive speed on the "radial extremes."

Hey everyone! As part of Christmas unwinding, I worked on my C1541 floppy audio drive some more. I shot two videos of it where I show lots of details in case anyone in the future wants to work on a C1541.

Here's the first one, where I show a variation from the schematic:
http://youtu.be/H4aVMcKC4as

And here's the second one, where I play with the speed control potentiometer!
http://youtu.be/CczRqEiD7FE