I'd like to build a (micro)cassette tape unit

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Y888099
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I'd like to build a (micro)cassette tape unit

Unread postby Y888099 » Fri Apr 07, 2017 3:48 am

So, in my to-do-list, I have it, since I'd like to build a cassette tape unit, with a digital interface for data.

Someone happens to have a schematic? Especially I'd like to see the pre-amplifier connected to the tape-read-head, and the modulator connected to the tape-write-head.

Every tape head are made from rings of ferromagnetic material with a gap where the tape contacts it so the magnetic field can fringe out to magnetize the emulsion on the tape, so every tape head itself is just a electro-magnet, apparently (in the theory) it's a piece of cake which can be driven as reversible machine, I mean the same head can be used for recording and playback, but the practice is different, since the tape-read-head has higher impedance than the tape-write-head, and needs to be handled differently.

The cassette tape-read-head is the part of the tape transport mechanism that's responsible for 'reading' the information off of the magnetic tape. *Reading* may/might be a bit misleading. It's not like a laser pickup in an optical media device such as CDs, the information is stored as varied levels of magnetic density on the particles of magnetic material on the surface of the tape, and the *tape-read-head* in cassette tape player converts the fluctuations in the magnetic field on the tape into an electrical A.C. waveform. The output is at a very low voltage. A pre-amp equalizer is the first circuit after the tape-read-head. This is not the type of equalizer that you'd have in or under your head unit. It's a specialized pre-amp that changes the level of various parts of the audio spectrum to allow the tape player to produce a relatively flat frequency response. If a sine wave sweep from 20-20Khz was accurately recorded onto a cassette tape, the audio head would not be able to accurately reproduce the correct voltage level across the sweep of frequencies, especially at the lower frequencies. The equalizer circuit is employed to *flatten* the response of the audio output and therefore produce a more accurate output. BTW, FSK modulation uses only two tones at different frequencies, which makes the equalization simpler.

The cassette tape-write-head is a bit more complex. Cassette machines that can record have at least two heads. The first head erases the tape just before the second head plays or records the new material. For a machine with only two heads, you have to record the material, rewind it and then play it to know how the recording sounds. Not a problem for me, but the tape-write-head needs a over modulator to compensate the physical gap

Getting the drive level right will be tricky. I don't know how DDS works internally, I am tempted to use a simple approach for the encoding and decoding: the pretty old school of the FSK modulation with a tone for '0', a tone for '1', and a PLL to distinguish them, and the higher layer of the interface will look just like a serial port at 1200bps without handshake. And something like having 64Kbyte of storage on a 90 minutes tape :D

Micro cassette are so cute, even if the bandwidth is limited to 5-6Khz (on a common audio cassette it's is 10-20Khz, two tracks), and you can have only one track on the tape.
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Re: I'd like to build a (micro)cassette tape unit

Unread postby johnnym » Fri Apr 07, 2017 6:52 am

If I understand you correctly, you want to build a Datasette? I believe there might be a lot of information available about these devices. The References section of the Wikipedia article could be a good start:

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Re: I'd like to build a (micro)cassette tape unit

Unread postby Y888099 » Fri Apr 07, 2017 7:34 am

Interesting articles, thanks.

johnnym wrote:If I understand you correctly, you want to build a Datasette?


Yup :D

I have already built the interface to a Sony Walkman Cassette Player & Recorder. This interface takes data from the serial port and puts them into FSK form (two tones, frequency modulation), so this interface looks like a modem. The serial RS232 line on one side, two audio jacks on the other side. A PLL does the inverse job, getting tones from the tape and recognizes '1's and '0's.

Next step: replacing the whole Audio Walkman with a circuit which directly interfaces the two tape-heads as well as the tape-motor.

Difficulties: the pre-amplifier (OA) directly connected to the tape-read-head is not a piece of cake, getting the drive level right will be tricky, and the the tape-write-head needs a modulator.
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Re: I'd like to build a (micro)cassette tape unit

Unread postby guardian452 » Thu Apr 13, 2017 4:19 am

FSK? Why not QPSK?

Cassettes have couple khz of bandwidth, I think you can get the storage a lot higher than 64kb on a c90 :)

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Re: I'd like to build a (micro)cassette tape unit

Unread postby Y888099 » Thu Apr 13, 2017 9:19 am

guardian452 wrote:FSK? Why not QPSK?


Because it adds more complexity and I need to be focused on the tape-head's interface.

guardian452 wrote:Cassettes have couple khz of bandwidth


MicroCassettes have 4Khz of useful bandwidth, and it's not flat due to all the strange properties (=not linear in frequency) of the magnetic tape.
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Re: I'd like to build a (micro)cassette tape unit

Unread postby guardian452 » Fri Apr 14, 2017 7:12 am

Yes, first you get it working, then you make it "gooder" :D

IIRC (I am no expert) that qpsk is around 2bit/sec per Hz of bandwidth. So 4khz bandwidth could be up to 8kbit/sec 60kbytes/minute or 5.4 megabytes on a c90.

I'm wondering how close that is to reality.

Next step: cutting data tracks in vinyl :twisted:

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Re: I'd like to build a (micro)cassette tape unit

Unread postby Y888099 » Fri Apr 14, 2017 7:43 am

Yup, I am also studying solutions implemented on floppy and hard drives.
They have a long synchronization frame. There are also three different methods to encode it:
  • FM, used on the first generation of floppies (8", 5.25")
  • MFM, used on the second generation of floppies (3.5"), and on the first generation of hard drives
  • RLL, used on modern hard drives

I like the simplicity of FM, but it's not optimal.
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Y888099
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Re: I'd like to build a (micro)cassette tape unit

Unread postby Y888099 » Tue Apr 18, 2017 12:48 pm

Image

Apart connectors for an external datasette, the HX-20 also offers built-in mass storage with the optional H20MC microdatasette cartridge, which accepts standard microcassettes used in dictaphones. The beauty of this gadget is that it's completely operated via software. Saving and loading is accomplished automatically -- no need to push play or record. The HX-20 maintains a 16-bit digital tape counter which is accessible from BASIC. A really nifty feature is the WIND command to automatically seek a specified tape counter position (usually only found on VCRs). A manual tape mode can also be selected to wind the tape via the function keys while displaying the counter. The microdatasette is also reliable (it actually saves each data block twice just to make sure!) and reasonably fast at 1300 bps. Simply put, this datasette puts the fun back into datasettes -- it's more entertaining than floppies!


Beautiful :)
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