Using a modern PC or other digital audio recording device as a cassette recorder replacement

Question

I'm having a hell of a time getting this to work. I've got a Casio fx-720p + FA-3 cassette interface, a TRS-80 PC-3 with its cassette interface and printer dock, and a Tandy 102 with a cassette cable.

I've tried hooking all three of these to both a modern PC running Audacity, and a small Philips digital audio recorder that's equipped with a mic input and records as WAV (i.e. no mp3 compression artifacts). On all of the computers, I can SAVE/CSAVE a program, and get an audio file that sounds like I would expect, i.e. lead-in tones, and separate bursts of data that sound like a file header and program data separated by a brief pause.

I've also checked continuity of audio cables with a multimeter.

Try as I might, I can't get any of these computers to successfully load or verify the program that was just saved. I've tried a wide range of volume settings (the Philips recorder has 15 discrete volume levels, and I tried every single one with the Casio). The Tandy 102 passes the audio input through to its internal modem speaker when the input volume is above a certain threshold, but it juts sits there with the "cload?" command on the display, never reporting any success or failure.

Do I need to be concerned about the recording volume level? The Philips recorder just has "high" and "low" sensitivity settings, which I'm guessing act as a coarse mic gain (either that or "high" sensitivity just compresses the dynamic range by boosting the low end; I haven't tested extensively, though I definitely hear a slight loudness difference between the two settings). What should the graph look like in Audacity? Just shy of clipping at +/-1.0? Hovering around +/-0.5?

I even tried hooking the Casio to my record player/cassette deck stereo system, using a sealed new-old-stock cassette tape, with similar (i.e. unsuccessful) results. I could hear the recorded data, but not actually load it.

What are the tricks to making this work? I would expect these computers to be pretty forgiving about audio, given the immense variability in quality and speed/pitch regulation of the average 1980s tape recorder, but even near-flawless digital recordings are getting me nowhere. Does the fact that these are all mono plugs being used in stereo jacks have anything to do with it?

FOLLOWUP: I acquired an adapter to split a 1/8" stereo jack into independent left/right 1/8" mono audio jacks. Still nothing from the Tandy 102; I've yet to test the other machines with it.

Answer 1

I am just guessing here (as I am used to ZX platform instead of yours).

1. MIC input

   that is not a good idea. Mic input might have a band pass filtering circuitry that could mess up with the serial feed of yours. Also the coupling is different as Mic is either passive component which requires source current from the recording device or an active one with very low power output and requires significant amplification. Sound output from a computer has diametraly different properties.

   Much better option is to use LINE IN input. Also proper coupling might need some additional circuitry to ensure voltage ranges and removal of the DC component. So you should check the manual/datasheets for both old and new device to crosscheck if they use the same voltages or not. If not add a voltage divider or amplifier depending on the difference.

2. Digital recording

   You need to chose sample rate at least twice as high than the top frequency recorded/used/probed. For example on ZX IIRC 35 KByte file was loaded in cca 5 mins. Just to be safe assume that 11 bits per BYTE was transfered (start/stop/parity) which will give us estimate of:

   35*1024*11 / 5*60 = 1314.1333 Baud
   

   So the sample rate should be at least 2630 Hz. Now depending on the loading routine on the target platform (how sensitive it is to timing...) the frequency needed might be even higher.

3. Output

   Well the old computers where build to use the more powerful outputs of tape player (of that time) so using Speaker output should be fine. Sometimes even headphone output is good but not always. LINE OUT will probably have not enough power to be usable without additional amplifier.

   Beware that some devices have playback effects like 3D sound, dampening of noise, emphasize speech, etc. Each of these can mess up the serial data and you should disable them if you can.

4. How should the recording look like?

   Not sure on your platform. I would however use oscilloscope on the output from your old computer during SAVE and then compare that to what is coming from your digital recorder during playback.

   In case you do not have an oscilloscope available You can use your PC sound card as one. In the past when I did not have an oscilloscope and needed one I decided to code one at least for Low frequency signals like this and here the result:

   • plotting real time Data on (qwt )Oscillocope

   In the link there you will find Win32 executables for Oscilloscope,Generator and Spectral analyzer using preferred sound input device in Windows (You can change it in control panel).

   I think the frequencies used should be more than enough for your purposes.

   Beware that volume settings is crucial for playback. Too low can dampen the signal beond recognition and too high can damage the dynamic range of signal up to the point it saturates to single voltage value.

   Here is the output from my ZX emulator seen on the Oscilloscope I linked and captured by my GIF capturer (only 2 fps so the file fits in here):

   

   As you can see it is a sharp rectangular signal (the spikes are due to coupling inductance) so you should see something similar ... I used LINE IN input of my sound card to connect the signal coming from speakers output (using stereo cable). Do not forget to set the default input device for recording in control pannel/sound & devices ...

Answer 2

You mention using CLOAD? That is normally used to verify that the file saved on tape is identical to the one in memory (i.e., to verify that the program can be loaded). You should just use CLOAD

For the Model 102 give this page a look: http://munk.org/typecast/2014/07/25/file-transfer-for-trs-80-model-100102200-using-a-digital-audio-recorder-the-road-less-travelled/

It has a link to a known good .wav file and also mentions that Play Cas and the reverse direction wav2cas understand the Model 102 cassette format at the byte level. They can display the data in a .wav file but don't know how to interpret it as a BASIC program.

I've used Play CAS quite a number of times to play cassette files to my TRS-80 Model III. I just hook the TRS-80 through the headphone jack. Generally speaking the volume wants to be about 50% on the PC. The article I mentioned about suggested that close to 100% volume was needed.

I've also done the same thing in the past with the TRS-80 PC-1 and PC-2 pocket computers. As I recall their tape format was quite a bit different (more like the "Kansas City Standard" and definitely was not compatible with the TRS-80 Model 1 or 3. I would guess that the PC-3 uses a similar format.

As an aside there many other ways to transfer files between a PC and a Model 102, especially with its RS-232 port. But I'll stick to the question at hand for now.

Answer 3

I found this question because I was getting the same symptoms on my Tandy Model 100 when trying to load a text file that I had just (seemingly successfully) recorded to an MP3 file on a Zoom H1 recorder - the TEXT program would just sit there and wouldn't respond to anything, even turning the unit off and on didn't help and I had to reset it (didn't lose any data, luckily).

In my case, the problem was I hadn't given any name when saving or trying to load, but just CAS: - when I saved the file again, but typed something proper like MYTEXT instead of CAS:, the recording of this could successfully be loaded again entering MYTEXT as the filename to search for. I have the manual for the Model 100, I just hadn't bothered to look this up before trying... Load/Save in TEXT always refers to cassette files, and cassette files must have a name. BASIC is different, it can LOAD/SAVE/OPEN various devices that you specify together with the filename.