By Carl M. Evans
Tale Of Two Circuits
Last time we discussed cassette tapes for the ATARI 410 Program Recorder, and concluded that mediocrity is best when choosing a brand. This issue we will look at the 410 Recorder itself, and explain how data (your program) is transferred from the computer to the recorder and back again. I'll explain why you can't use just any cassette recorder with your computer, and describe the amazingly simple, yet subtly sophisticated design of the ATARI 400 / 800 input / output for cassettes.
The 410 Recorder is a stereo (twotrack) machine that has been specially adapted for use with the ATARI 400/ 800 home computer. The usual tone and volume controls have been eliminated and these values preset at the factory. The controls left for the user are RECORD, PLAY, REWIND, ADVANCE (fastforward), and STOP / EJECT. Some models also have a PAUSE button. The interface cable is permanently attached to the recorder and has a sealed connector on the other end. This connector plugs into the PERIPHERAL opening on the computer, or into the 850 Interface Module, or into the back of the 810 Disk Drive.
If you looked inside the recorder's plastic case, you would see a capstan drive mechanism, a small transformer and a circuit board. This board is the heart (or heart-break) of the 410 Recorder. The size of the board varies in some models, but in mine it measures five-by-five inches. There are only a few dozen electronic parts on the board, barely enough for a board a third that size.
Atari does not publish its schematics so I had to trace out the board myself. In the next issue I will show you this schematic, and tell you how to improve the Recorder's reliability.
There are two separate circuits on the board. The first is for handling the "record" function. This is a simple circuit much like it would be in any cassette recorder. It works reasonably well and is not to blame for most cassette loading problems. Loading problems are almost invariably caused by the "playback" portion of the circuit board. We will go into this topic in more detail next time.
The POKEY chip in the ATARI computer generates a "pure" FSK signal that varies in frequency to represent Zeros and Ones. These tones are recorded on the digital data track of the tape with only moderate distortion. When you load a program into your computer from tape, however, the tones have to be converted to a binary serial data stream before the computer will be able to receive and understand them. This translation process is the primary function of the second circuit on the board, the "playback" circuit. Among other things, playback passes the tones on the tape through a couple of filters that are sensitive to specific tones and will convert them to the appropriate serial data values. These filters operate on a window, or bandpass, technique that is supposed to react only to the specific frequencies involved.
Most loading problems can be traced to some deficiency in this translation process. The filters in the 410 Recorder are not precision filters and the recorded tones are not necessarily pure by the time the playback circuit sees them.
Why can't you use just any cassette recorder with your computer? A normal cassette recorder, even an expensive one, is designed only to produce output similar to its input. If you recorded an FSK signal on one of those recorders, you would only be able to get an FSK signal out of it. Since your ATARI expects to receive a serial binary data stream, the computer would never recognize the data. The only way you will ever be able to use a normal recorder with your ATARI is if someone designs a translator that converts the FSK signals into the appropriate serial binary data.
Because of the lead time necessary in magazine publication, I am writing this article without really knowing what you readers are having problems with. Until I hear from you I assume that most of your problems are similar to those that I have experienced. If you have had a particularly aggravating problem with your cassette system, write to me in care of ANTIC and I will try to give you a helping hand.