YEKTA@mit-mc.arpa (03/09/83)
From: Yekta Gursel <YEKTA@mit-mc.arpa> Here is some reasons and solutions for tape loading problems people might be having with almost any computer: (a) First of all; the signal that comes out of the computer is nothing like a sine wave; usually they are square waves of frequencies 1200 Hz and 2400 Hz. (b) When these square waves are recorded and read back from the tape, teh waveform that comes out of the tape recorder is nothing like a square wave. The reason for this is that a square wave has a lot of high frequency harmonics, and tape recorder amplifiers have severe attenuation at high frequencies. (c) This is where the "tone" control on the recorder becomes important. Some cassette recorders have bad high frequency response. ( For recorders in the price range $35 to $100, the price is usually NOT correlated with the high frequency response; a cheap recorder might have a better high frequency response, believe it or not! ) What this does is to give back something that resembles a sine wave. Usually , the tape input ports on cheap vcomputers are happy with a sine wave; because they have a pulse shaper at the input. SO, everything works fine. If your recorder has a better high frequency response; depending on the frequency "roll-off" curve, the output will have a waveform that resembles a sine wave with ripples superimposed on it. Poor computers input port thinks these ripples are data, and hell breaks loose. The reason why the "tone" control helps this is that when it is turned towards "bass" it makes the high frequency response worse, thereby eliminating those annoying ripples. Since the ripple amplitude is proportional to the volume control also, usually a reasonable combination of these controls works. One might get a tape recorder where there is no "reasonable" setting of those two controls. That is where the job starts becoming hopeless. Remember also that the "tape" itself also contributes to this. (d) There is a neat solution to the problems above: If instead of outputting square waves, your computer outputs sine waves; then almost all of the problems above will go away, unless you have a really uncooperative tape recorder. SInce almost no computer does this; one might try to make the waveform more like sinusoidal BEFORE it gets recorded on tape. One (very cheap ) way of doing this is to connect a capacitor ( about 1uf, but one has to experiment, the value will depend on the computer. Do not get discouraged, this is VERY easy... )accross the tape output port. This does not convert a square wave to an exact sine wave, but it comes very close. It eliminates the high frequencies present very effectively. This method worked wonderfully in my AIM 65 for example. Now, I do not have to use the tone control at all, and the damned thing works at almost any setting of the volume control as long as the signal amplitude is not too small. I hope this will help some of you enjoy your computer more, whatever it might be... Best, Yekta
steve (03/14/83)
I just thought I would mention that the TRS Color Computer has a 6-bit D/A inside and one of the things it does is synthesize sine-waves for recording on tape - thus making the tape save/restore mechanism EXTREMELY reliable. I guess on the fourth time around the Tandy engineers finally figured out how to do it right.