Analog Tape Machine Alignment (11)

The Music Telegraph | Text 2021/03/09 [11:04]

Analog Tape Machine Alignment (11)

The Music Telegraph| 입력 : 2021/03/09 [11:04]

 

© Wikimedia Commons



Analog Tape Machine Alignment (11)

 

(Continued from the previous article)

 

We have aligned the electronics for flat response, got the record bias just right, now let's find the dynamic range and noise floor of your analog tape machine.

 

What we are going to do is make a recording of the signal generator's output as you raise the level of the signal going into the tape machine. At some point you will begin to see a change in the wave shape on the oscilloscope. It will change from a smooth sine wave to something that is not smooth. What the actual shape is not important just yet. The point is that it is not the sine wave that went into the tape machine, hence it has become distorted by the tape recording process. We want to know what this signal level is because it is the upper limit of allowable signal (the maximum output level of MOL) for this machine's speed/tape formulation. The ACVM (AC Volt Meter) will allow you to read the level of this signal.

 

Once you have determined the MOL, you will then remove the signal entirely from the tape machine and continue to record. Whatever is coming out of the tape electronics at this point mustbe due only to the recorder's electro-magnetic systems. This is what we commonly lump together as tape hiss. This represents the machine's noise floor. Again, the ACVM will allow you to read the level of this signal.

 

Armed with your MOL and noise floor measurements, you can easily calculate your tape machine's dynamic range for this tape formulation and speed. 

 

Note: If you run out of tape...

 

All the measurements that follow must taken while the tape machine is in record mode. If you run out of tape, simply rewind to the beginning and start recording again (or flip the tape over and start recording again).

 

 

1) Put the tape machine into record and look at the image on the scope.

With any luck at all you should be seeing a few cycles of a sine wave.

 

2) Gradually raise the output level of the signal generator until you just begin to see the sine wave distort (change shape).

You may have to readjust the scope's vertical control to keep the signal inside the screen.

 

3) When you have just the beginning of distortion on your scope, read the ACVM and enter the value below.

 

(example)

Maximum Output Level (MOL) at 15 ips @ 250 nW/m is 10 dBv

 

 

4) With the tape machine still in record mode, turn off the power to the generator.

  

5) Read the ACVM and enter the value below.

 

(example)

Output Level with no signal input (noise floor) at 15 ips @ 250 nW/m is -22.5 dBv

 

 

6) Stop the tape machine.

 

7) Calculate the dynamic range of your tape machine set up.

This is real simple. The dynamic range of the system is simply the difference between the MOL and the noise floor measurements. Fill in the blanks and you'll have it.

 

(example)

MOL:  +10 dBv

(-) Noise Floor:  -22.5 dBv

_____________________

Dynamic Range:  32.5 dB

 

 

 

 

 

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