Measuring the frequency response of tape playback electronics

[Fred Thal]

For an end user of a tape repro electronics to do this, we'll assume the use of a reproducer alignment tape, a so-called calibration tape.

Yet a limitation with the popular cal tapes is the too sparse distribution of frequencies.

So, if you were a qualified designer of tape electronics (or someone seriously wanting to investigate the performance of any tape electronics) you wouldn't accept only 13 discrete spot frequencies to examine the response because you wouldn't get an accurate picture. You'd insist on about ten times as many! (We're talking one-twelfth octave resolution.)

Is anyone thinking: Wouldn't this measurement take all day to accomplish?

No, it would take under two minutes.

Actually, under 25 seconds for a 20kHz to 20Hz, 112-step sequenced test signal, using this 35-year-old hardware, with its specified cal tapes (from MRL):



Well, pardon the big marketing sign. I didn't want to edit the image. The (quickly obtained) resulting plot on the ST 1510A is saved and examined.

Or, printed and shipped with the product as its proof of performance.

Which of the newer tape electronics manufacturers are doing this?

(The respected older ones did.)

 

[microstrip]

(...) Which of the newer tape electronics manufacturers are doing this?

(The respected older ones did.)

They could easily do it if they wanted - any inexpensive modern soundcard based audio spectrometer can record and analyze such signal.

The real advantage of the 1510A is that for an experienced user it can be faster and more efficient - just one box, optimized for these particular tasks. But currently inexpensive virtual instruments can be very powerful.

BTW, audiophiles are usually only concerned with playback of pre-recorded tapes.

 

[Fred Thal]

They could easily do it if they wanted -

The real audio people here already know that when plugging in something new (like a tape stage) and everything immediately sounds very different, the first thing to check are the level and response matching between the two units being compared. Only if you do this first, can you then make meaningful evaluations about sound quality differences.

This is easy to do with tape, fortunately!

That's what this thread was going to be about, but no one cares?

The first Studer A820s that I saw modified for switch-controlled selective use of internal or outboard audio electronics, were owned by one of the Big Three, here in the States. This was 25 years ago. The mod was actually competently engineered and executed, done by people who fully understood what they were doing. I took a lot of notes.

 

[christensenleif@msn.com]

I care and would like to hear the rest.....
best
Leif

 

[topoxforddoc]

Fred,

If I want to do this properly, what else do I need apart from the ST1510A plus suitable MRL tape? It seems quite hard to find a ST1510A in the UK or Europe. Is there another machine I could use instead?

Thanks

Charlie

 

[Fred Thal]

Fred,

If I want to do this properly, what else do I need apart from the ST1510A plus suitable MRL tape? It seems quite hard to find a ST1510A in the UK or Europe. Is there another machine I could use instead?

Thanks

Charlie

Hi Charlie and Leif,

You definitely do not need to go try to find an old ST 1510A.

microstrip (sorry, do not know his or her real name) who posted to this thread had it exactly right.

If you don't mind an excursion into the digital domain, the instrumentation available today for audio signal analysis is amazing in its accuracy, its versatility and its low cost. Bottom line is for audio testing DSP is your friend!

The truth is, I only keep the old 1510As on line here for the sheer fun of it. (It's an early Zilog microprocessor based instrument!)

On the Audio Science Review forum today, Amir is beginning to investigate how to properly measure tape playback electronics using MRL cal tapes. Rather than try to duplicate the postings from that forum here, I suggest you plan to read the goings on over there.

Unfortunately, it's in a thread called Phono Pre-amp: What are we testing for?

I think I'll suggest a new thread title there.

 

[microstrip]

Fred,

If I want to do this properly, what else do I need apart from the ST1510A plus suitable MRL tape? It seems quite hard to find a ST1510A in the UK or Europe. Is there another machine I could use instead?

Thanks

Charlie

Even if you manage to get these old measuring machines, it is very hard to keep them working properly and calibrated. If you are an occasional user I would suggest an USB good quality ADC/DAC (yes, I know it is an heresy to use such words in a tape thread ) - even some of the inexpensive are accurate to .1 dB and some software with recording and processing capabilities. I like SpectraPlus, but I have seen people using several others.

 

[topoxforddoc]

I have a Focusrite Scarlett 2i2 usb audio interface, a Mac and Audacity. Is this what I have to do?

1) Plug the outputs from the R2R into the XLR inputs on the audio interface
2) Launch audacity from the interface on my Mac
3) Play a test tape, whilst recording on audacity
4) Then review this on audacity with the analyse - plot spectrum

Thanks

Charlie

 

[topoxforddoc]

Hi Fred,

Have i got this correct?

Thanks

Charlie

 

[Tapetech]

What you need to do depends on which MRL tape you have. You also need to tell which deck this is for and the speed/EQ you will be adjusting for.

 

[Tapetech]

If I want to do this properly, what else do I need apart from the ST1510A plus suitable MRL tape? It seems quite hard to find a ST1510A in the UK or Europe. Is there another machine I could use instead?

As Fred pointed out, you don't need an ST1510A or any other machine to generate a plot of the playback frequency response. You need the appropriate software and MRL tape. I use AudioTester V3.0 and an MRL tape that has a 20 to 20K sweep tone. See graph of my Tascam BR-20 playback response. View attachment 41142

 

[Ki Choi]

I use ST1510A's but the fans in them are driving me mad. What 2 channel quality sound card are you using?

 

[stellavox]

Ki,

I put a resistor in series with the fan to quiet it down - can check the value/wattage if you'd like

Charles

 

[Tapetech]

I am using a Focusrite USB interface.

https://us.focusrite.com/usb-audio-interfaces/scarlett-2i2

Here is another 20 to 20K playback FR plot using my new tape preamp, Flux Magnetics head and Technics transport.

View attachment NewpreampsweepFMhead.pdf

 

[microstrip]

Still using the old EMU tracker. http://www.creative.com/emu/products/product.aspx?pid=17511

The nice extra is that it includes a 48v phantom supply for the ECM8000, needed when I want to use it with REW for acoustics.

 

[squasher]

As Fred pointed out, you don't need an ST1510A or any other machine to generate a plot of the playback frequency response. You need the appropriate software and MRL tape. I use AudioTester V3.0 and an MRL tape that has a 20 to 20K sweep tone. See graph of my Tascam BR-20 playback response. View attachment 41142

1) Now that AudioTester is no longer a valid option (since the developer has gone MIA), do you have an inexpensive piece of software that will do the same job?
2) Which MRL tape is the one that contains the sweep tone? Is it the same one made to work with the Sound Technology machines?

 

[Tapetech]

You can try TrueRTA. It has a quick sweep function for testing rec/play response.. It will not generate a plot from the MRL sweep tape, however (like the one I posted).

I had an email conversation with the AudioTester developer back in May and he made no mention of his site closing.. See if you can contact him about the site closing down. Name is Ulrich Mueller. email is info@audiotester.de. FAX is +49 202 2712400

Here is where I bought the MRL sweep tape from. It has also test 5 tones on it if you don't have software for plotting the sweep tone. It is not the sweep tape used for the Sound Technology device.

https://www.ebay.com/itm/MRL-1-4-NE...886061?hash=item3f0318a3ad:g:j64AAOxylhxRF92C

 

[DexterMiller]

A "Rube Goldberg"/"on-the-fly"-type way of doing this years' ago, could've been rigged-up by: using a FM signal as a reference point while biasing a blank and a/b'ing the output between the tape being recorded vs. the source monitor; where (presuming the record and the repro heads were already properly aligned as well as demagnetized), you'd compare (all by ear) whether the deck had the ability to register:

1. The 15khz top-end limit of a mono FM reception (where there should be no static artifacts audible at this level).
2. (With a MPX stereo signal): whether the deck could capture (now) the very definite change in high pitched hiss added to the signal as well as faintly detect interference from the 19khz FM stereo pilot (something the 90volt record amp on a vintage deck as mocked as a Dokorder can pick up).
3. (When recording at 15): a single-motor, moderate-grade deck from 1972 (and one of the few one motor decks to have included 15 with dedicated equalization adjustments), a Panasonic "RS-736"...even displayed the specs to record -while running at 15- the 30khz subcarrier frequency of an old quadraphonic CD-4 decoder (which is pretty amazing capability for an idler drive unit with pressure pad head geometry!).

Of course, I wouldn't go about it this way when in the presence of $20k machinery...but it was an interesting experiment way back.