It's likely all to do with the expected input impedance of the old fashioned tape recorders which use "tape heads" which are basically coils which are inductors, which causes a much different impedance than plugging into your computer line in. Thus, the caps and resistors on the output circuit are spec'd for that expected impedance. It's analogous to plugging in a guitar output to a microphone jack. It's not the right impedance and thus the load voltages are different, some frequencies are attenuated, etc.
The 47pf/1k rc filter is probably just cutting out any high frequency overtone anomalies above 20k, as well as matching impedance with the expected tape deck (as well the 1M ohm resistor), and the 47nf is a bypass cap for any stray rf on the signals (via tv or IC's). Just guesses though. If you want accurate per intended spec, measure the signal while under load of a tape deck. Probably more accurate than if you run straight off of ULA pin 16 and get a measurement which voltage-wise matches the voltage measured going into an old tape deck (via a potentiometer) which may not have matching amplitudes overall. If you use a preamp you're in the same boat... different impedance, different output than a reactive tape head.
Very low signal on MIC port
Re: Very low signal on MIC port
RP55 - not quite, the MIC input of these old cassette recorders doesn't directly feed the record/play head, but usually an automatic gain controlled pre-amp circuit before feeding the head, and is usually lower impedance than that of a 'LINE' or 'AUX' input, a Tandy CTR55 that I have has a 1K resistor from MIC in to ground, before feeding the amplifier chip, and that machine worked well with a zx81 and spectrum, until the belt stretched too much!
Re: Very low signal on MIC port
Hi gozzo,
I appreciate your response very much as it got me thinking... In my mind I want to say you are correct per many people mentioning "just put this at the beginning of your input circuit to change impedance". But doesn't the total impedance formula say otherwise:
impedance = resistance + (sqroot of -1) * (inductive reactance - capacitive reactance)
Obviously, reactance is frequency dependent factor on the frequency output in question... given inductive and capacitive components in both circuits. These are mathematically comprehensible assuming simple serial and simple parallel circuits are involved, of which they are not and I am not yet at the level of calculating such via more complex known techniques (you may be and I wouldn't doubt that).
Excluding the j thing (sqrt of -1 factor) for a moment (perhaps you can clear that up for me); impedance as a frequency dependent factor with respect to ohm's law, given the above formula, is not minimized with more resistance, is it? It is merely amplified (no pun intended). And perhaps that amplification is what is enough for the "just add component x at the front" to be accepted as making such input impedance negligible. I'm all ears for corrections on this understand if you got it down.
Anyways... given that... the most accurate thing is to put this on a circuit load which was representative of the period. And from a "creating an emulator" standpoint... perhaps deciding how much difference that actually makes in the end product.
I'd love to understand how adding resistance to this circuit lowers the impedance given the total impedance formula from what you understand. I'm not saying you don't know... just interested in learning (because there's so much I don't understand still!).
From Wikipedia on Electric Impedance:
"The measurement of the impedance of devices and transmission lines is a practical problem in radio technology and others. Measurements of impedance may be carried out at one frequency, or the variation of device impedance over a range of frequencies may be of interest. "
Perhaps it's that nasty j factor (sqrt of -1) I haven't gotten around to understanding yet!
... and in just thinking about that j factor, it is a number very close to 1 (per calculus slope derivatives I would imagine) and since it is negative it is a way to ensure that the reactance outcome used in the formula is always a little less and negative than the total reactance (for reason not yet understood). Even so the resistance minus a negative value of total reactance means that total impedance (including resistance) will increase with less reactance and decrease with more reactance... I think... lol. But the bottom line in my mind is that more resistance is clearly going to cause more impedance, not less.
The most challenging question from a "creating an emulator" standpoint in my mind would be being able to successfully output the signal via the emulator and read it back in via a real machine. And the reciprocal: being able to read in to the emulator a machine output signal. Therein lies the most pragmatic problem involved... and exceeds merely accurate reproduction of the original via the given circuit. The ULA is apparently somewhat forgiving... and so must the aforementioned scenario. How forgiving the chip is may be not enough to provide this. Any make up will have to be realized into the emulator to float in the acceptable midway point, if possible.
I appreciate your response very much as it got me thinking... In my mind I want to say you are correct per many people mentioning "just put this at the beginning of your input circuit to change impedance". But doesn't the total impedance formula say otherwise:
impedance = resistance + (sqroot of -1) * (inductive reactance - capacitive reactance)
Obviously, reactance is frequency dependent factor on the frequency output in question... given inductive and capacitive components in both circuits. These are mathematically comprehensible assuming simple serial and simple parallel circuits are involved, of which they are not and I am not yet at the level of calculating such via more complex known techniques (you may be and I wouldn't doubt that).
Excluding the j thing (sqrt of -1 factor) for a moment (perhaps you can clear that up for me); impedance as a frequency dependent factor with respect to ohm's law, given the above formula, is not minimized with more resistance, is it? It is merely amplified (no pun intended). And perhaps that amplification is what is enough for the "just add component x at the front" to be accepted as making such input impedance negligible. I'm all ears for corrections on this understand if you got it down.
Anyways... given that... the most accurate thing is to put this on a circuit load which was representative of the period. And from a "creating an emulator" standpoint... perhaps deciding how much difference that actually makes in the end product.
I'd love to understand how adding resistance to this circuit lowers the impedance given the total impedance formula from what you understand. I'm not saying you don't know... just interested in learning (because there's so much I don't understand still!).
From Wikipedia on Electric Impedance:
"The measurement of the impedance of devices and transmission lines is a practical problem in radio technology and others. Measurements of impedance may be carried out at one frequency, or the variation of device impedance over a range of frequencies may be of interest. "
Perhaps it's that nasty j factor (sqrt of -1) I haven't gotten around to understanding yet!
... and in just thinking about that j factor, it is a number very close to 1 (per calculus slope derivatives I would imagine) and since it is negative it is a way to ensure that the reactance outcome used in the formula is always a little less and negative than the total reactance (for reason not yet understood). Even so the resistance minus a negative value of total reactance means that total impedance (including resistance) will increase with less reactance and decrease with more reactance... I think... lol. But the bottom line in my mind is that more resistance is clearly going to cause more impedance, not less.
The most challenging question from a "creating an emulator" standpoint in my mind would be being able to successfully output the signal via the emulator and read it back in via a real machine. And the reciprocal: being able to read in to the emulator a machine output signal. Therein lies the most pragmatic problem involved... and exceeds merely accurate reproduction of the original via the given circuit. The ULA is apparently somewhat forgiving... and so must the aforementioned scenario. How forgiving the chip is may be not enough to provide this. Any make up will have to be realized into the emulator to float in the acceptable midway point, if possible.
Last edited by rp55 on Sun Jan 12, 2014 1:01 am, edited 1 time in total.
Re: Very low signal on MIC port
What the hell you are talking about ? 
Re: Very low signal on MIC port
lol I'm rambling on... my first mistake was going back to this complicated little door stop! Honestly, this little machine is reality.PokeMon wrote:What the hell you are talking about ?
Re: Very low signal on MIC port
The amplitude on all frequencies will be lower (across the spectrum) given any resistance (not reactance) on the input (to the tape deck) per gozzo's comment about the tape deck having a resistor to ground on input. But certain frequencies will still be affected per the reactive components both in the tape recorder circuitry and in the originating (sinclair) circuit. It doesn't matter how much resistance you add... the amplitude will still be (given the formula I went over) overall nothing but less amplitude, but within it it will still reflect reactance in both circuits. Perhaps I'm missing something but the impedance formula doesn't budge for me. Merely the addition of a resistor on the tape deck input to ground does not change the reactance within that amplitude reduction.PokeMon wrote:What the hell you are talking about ?
Now if you add capacitive components to negate the inductive reactances then that is something else. Those can equalize such impedance. But not resistors? I mean I'm all ears if I don't understand it. That's how you learn.
I guess I can understand that less amplitude (less coulomb) passing thru the caps (for instance) causes a variation in cap reactance on certain frequencies (different than the original amplitude). But the relative outcome on the attenuated waves due to reactance which are affected is the same, no? Less attenuation... but then there is less overall amplitude...
Re: Very low signal on MIC port
Good grief! 
RP55 - if what you are saying is any capacitive loading in the input circuit of the tape recorder will affect the impedance, you'd think it would - it is also loaded to ground by a 2200pf capacitor, which has a reactance of 3.6k at 20KHz - but which when combined makes an impedance of 0.96k (if I did the calculation correctly?!),so no real difference actually!! Only really starts making a difference when getting into RF frequencies, and may well be there to stop the pre-amp/amp/osc chip oscillating where it shouldn't! Another similar but different (if you get what I mean 
) tape recorder I have has a 1500pf capacitor to ground (but can't see any resistor..maybe internal to the chip?)...as for the output components of the ZX, that is what really shapes the output characteristics, and that is also loaded to ground by a 1k, but a much higher capacitance of 0.047uf, making a combined impedance to ground of 166 ohms at 20KHz and 860 ohms at 2KHz..now here's a variation, and it's fed through a 1Meg resistor in series with a 47pf capacitor (reactance of 169k at 20KHz, 1.69Meg at 2KHz)...which means, if I have worked it out correctly, the ULA's output is attenuated by 7043:1 at 20KHz and 3128:1 at 2KHz... if then loaded by the 1k of the tape input, it would change the ratios to 8291:1 at 20KHz, and 5823:1 at 2KHz....I think 
RP55 - if what you are saying is any capacitive loading in the input circuit of the tape recorder will affect the impedance, you'd think it would - it is also loaded to ground by a 2200pf capacitor, which has a reactance of 3.6k at 20KHz - but which when combined makes an impedance of 0.96k (if I did the calculation correctly?!),so no real difference actually!! Only really starts making a difference when getting into RF frequencies, and may well be there to stop the pre-amp/amp/osc chip oscillating where it shouldn't! Another similar but different (if you get what I mean ) tape recorder I have has a 1500pf capacitor to ground (but can't see any resistor..maybe internal to the chip?)...as for the output components of the ZX, that is what really shapes the output characteristics, and that is also loaded to ground by a 1k, but a much higher capacitance of 0.047uf, making a combined impedance to ground of 166 ohms at 20KHz and 860 ohms at 2KHz..now here's a variation, and it's fed through a 1Meg resistor in series with a 47pf capacitor (reactance of 169k at 20KHz, 1.69Meg at 2KHz)...which means, if I have worked it out correctly, the ULA's output is attenuated by 7043:1 at 20KHz and 3128:1 at 2KHz... if then loaded by the 1k of the tape input, it would change the ratios to 8291:1 at 20KHz, and 5823:1 at 2KHz....I think
Last edited by gozzo on Sun Jan 12, 2014 3:32 am, edited 1 time in total.
Re: Very low signal on MIC port
My apologies for the "grief". But if I wanted wishy washy I would have taken up a different interest now days. I like it tight now and with a minimum of subjection.gozzo wrote:Good grief!
You didn't mention any such cap earlier on; only a resistor. That certainly changes things now. I'm sure you would agree bringing that up initially might have alleviated my long response (per your strong disagreement with my initial one). Perhaps you didn't quite understand that then but do now (after my long explanation). Perhaps not. Certainly a time gap between both responses... it's possible. There again.. my non-subjective side coming out.
So given what you say there should be little difference in the apparent relative frequency attenuation due to the still existing reactance involved (which is still gradational throughout the entire frequency spectrum). Fair enough given a subtractive reactive component is now involved and not just a resistor.
Re: Very low signal on MIC port
edited my post , done some more calculations.. ^
Re: Very low signal on MIC port
Cool. I think it will be fun to see them and analyze them (and learn from them at least in my case). Just a side note thought... curious what your opinion is... what about the fact that the tv is in direct (not EMR) contact with the tv and its impedance (during tape load and display wierdness via the "feature" of showing wierd lines on the screen)? I understand that is on the other side of the digital realm within the circuit to some degree; on the other side of a digital buffer (at least within the ULA as it was). But its still analog. Do you think that had impact on Sinclair/Timex's original calcuations for the impedance matching (if any) that was done? Not sure if I'm way off base there and curious to hear your thoughts on that. I guess I don't know yet how IC's affect impedance. I would assume they are just resistance and impedance issues even on the other side of them will still be relevant.gozzo wrote:edited my post , done some more calculations.. ^