Theoretical Record Frequency Response

[Dan McDonald]

The other day I transferred some material that had originated on 78s. Notably, two specific things:

1. a Clarence Williams jazz record with Louis Armstrong and Sidney Bechet - originally recorded in 1924 and released on a compilation lp in the early 1970s.

2. a Dave Appolon Gypsy mandolin piece recorded originally in the late 1930s and released on lp in the late 1970s.

These were both recorded in the same file. I simply placed the record on pause and changed the lp. I looked at the transfer in the spectrogram and teh Clarence Williams piece's highest frequency is around 8-9kHz, which I think is consistent with the calculations Craig made below.

The Dave Appolon piece is way higher - 18-20 kHz, and I'm pretty certain this is real music (not artifacts) - there aren't any clicks at the point where these high points are reached, but he is hitting some very high notes here. I read the liner notes for the album and David Grisman was involved in the transfer to lp, saying that Dave Appolon's widow provided the master records for the transfer. These high notes are not at the beginning of the record, they're scatterred throughout, which means they shouldn't be that high, right?

I was just wondering it that meant that engineers had found some way around the frequency limitation problems by the late 30s.

The notes also mention that Dave A. had a trick for reaching the high notes on his mandolin - by removing the 26th fret, he could play the 27th. My mandolin doesn't even go that high, so I know he is hitting some very high notes when he's playing.

Dan

[Craig Maier]

Hi Dan,

I think that you may be ignoring the effects of non-linear systems on the production of harmonic distortion. Record recording systems contain a whole string of devices that have non-linear transfer characteristics, including the cutter head itself. All of these things will produce frequency multiples (odd and or even) of their respective fundamentals. Thus, they will appear as real modulated information on a spectrogram which follow the musical content.

If you want to experiment with this effect, the VVA is a great experimental science lab. Create a 1 KHz sine wave with the make waves generator and then check it out on the spectrograph. Then, run the VVA on it (the VVA uses Vacuum Tube models which are non-linear devices) and check it out on the spectrograph. You will see that the frequency content all of a sudden extends way past the 1 KHz excitation frequency. The same thing happens in the record mastering process for various reasons. Now, you may think that the VVA is just amplifying the high end. To prove that this is not the case, take the 1 KHz sine wave and run it through an EQ. You will see (so long as you do not clip the EQ) that the sine wave looks basically the same no matter where you set the EQ frequency controls.

Back in the 1970's, the only method that they had to reduce noise on a transfer was the use of a low pass filter / EQ. There were no sophisticated devices as found with the Diamond Cut suite. It could be that the out of band noise was reduced to a greater extent on the Taylor stuff when compated to the later stuff.

I hope that this helps in understanding the distinction between a systems intrinsic frequency response and its propensity to introduce harmonic (and intermodulation) distortion products into the signal pathway.

[Dan McDonald]

OK - but then a lot of that is 'pleasing' information, even though it is introduced by the system, isn't it? If so, then you wouldn't necessarily want to get rid of it, right?

My thinking is that there are a lot of harmonics, etc. produced naturally when you play an instrument. If the original recording medium wasn't able to capture them, but something in the process 'imitated' them or reproduced them, wouldn't you want to keep them in the file?

I was thinking that some of the posts in this thread suggested that everything above the 8kHz or so would be noise, but if it's a natural-sounding harmonic, I would think of it as 'true' information. Or am I thinking of it the wrong way? When you play an instrument, harmonics pop out all over the place. If you get them back in spite of the limitations of the recording medium, that seems like a good thing to me.

Dan

[Craig Maier]

Quoting Dan:

"OK - but then a lot of that is 'pleasing' information, even though it is introduced by the system, isn't it? If so, then you wouldn't necessarily want to get rid of it, right?"

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I can only say that these extraneous signals are not part of the original sound source signal. Sometimes they can be pleasing and sometimes not. It is a matter of the material coupled with a persons taste in sound.

As a point in example, look at the fact that some people will only listen to vacuum tube audio systems. These folks are always introducing around 5 % harmonic distortion into their signal pathway and they love it. On the opposite extreme, take myself. I use op amp based preamplifiers and commercial high powered solid state power amplifiers which introduce less than 0.01% harmonic distortion into the system and I love it.

My view is that you can always add the harmonic distortion into the signal pathway at any point in time, but it is next to impossible to get rid of the distortion (no matter how pleasing it may sound) if you so desire at a later point in time.

[Dan McDonald]

OK - that makes sense to me. I guess it just struck me as odd to see high-frequency signals from 78s.. and they certainly sound very natural. It seemed counter-intuitive to cut them out and then to try to restore something that approaches the natural sound of the instrument by adding harmonics back in with the VVA or the enhancer in the DNF. But it makes sense to me the way you explain it.

Dan

[Craig Maier]

If you like the way that they sound, then just keep them the way that they are. But, they are distortion products; 78's just do not have the capability to record much more than the math at the top of this thread demonstrates.

And, it is hard to argue with the math. I did not invent the physics of it all, I just set down the assumptions, derived the equations from the physics and then solved the equations under a number of different circumstances. I guess that it just is what it is unless there is an error in my assumptions.

[Brian]

Hi Dan:
You're right about the "pleasing" effect of some distortion products - just ask Dr. Bose! Actually, the Bose speakers are among the least accurate speakers on the market - but lots of people find their intentionally induced distortion quite pleasurable. Figure out the math on how to recreate the distortion you observed and you too are on your way to becomming a multi-millionaire!
Brian

[Craig Maier]

It should be very easy to measure the sonic deviations produced by such a system. Excite the speakers with random noise, and use a calibrated microphone into the Spectrograph or Spectrum Analyzer and you have their frequency response. Then, excite them with a swept sine wave and observe the distortion products on the spectrograph. If you are interested in characterizing them vs. loudness, contours can be measured at various power levels (commonly known as loudness countours).

Voila! The magic is revealed.

Note: Everything that you need to do this is found in your software but for the calibrated mic and an acoustically dead room. B & K (Brüel & Kjær) makes a good microphone for this kind of work. If your wife has a walk-in closet packed with clothes, that makes an excellent anechoic chamber.

[Dan McDonald]

Thanks Craig. I never doubted the math - just wondered what was going on with things. It was very clear in the two samples that there was something very different occurring.

Dan

[Craig Maier]

Another thing that just occurred to me is that this LP was mastered in the early 70's. Many audio preamplifiers of that era were vacuum tube based which produced much higher levels of harmonic distortion compared to todays models which employ op amps (operational amplifiers).

Also to consider is that the 1924 recording was probably acoustically mastered whereas the 1930's recording was mastered electrically. Naturally, the acoustically mastered recording started out with much less bandwidth compared to the electrical. Less top-end also means less distortion product harmonics being produced at the top end of the audio spectrum by the audio transfer system used back in the early 70's.

[Audyossey]

Bose

Hi Dan:
You're right about the "pleasing" effect of some distortion products - just ask Dr. Bose! Actually, the Bose speakers are among the least accurate speakers on the market...

Are they still? I'm quite familiar with the fallacies of the advertising hype by which they made their fame and fortune in the 80s, or whenever it was. And they still hype the low-end consumer stuff to death with a lot of nonsense. But the basic idea was always to "enhance" the low end of the bass drivers with electronic compensation, and that idea?more or less?is quite commonplace and expected these days, as for example, in low-priced "subwoofers." I would expect that contemporary Bose products are just quite ordinary products?not exceptional, not bad. But then I haven't been paying attention.

HB

[Geebster]

I can't speak of Bose's current production, but I know for a fact as late as '95, at least the series 901 speakers were still built like tanks. We used only 2 in a small night club and pumped 3000 watts into them on a nightly basis.

In a nightclub setting, accuracy isn't your main objective, but durability is.

GB

[DJBohn]

Hey Geebster,
I've always felt that the Bose 901's were best suited in a large bar or night club. I had the pleasure of using a pair once in a large party room in the basement of a Legion Club for a family benefit of a deceased friend. I was putting dynamic peaks of 300 to 400 watts into each of them & they took every bit of the power I could give them. They didn't sound too bad either, but the larger the room, the more the bass can be emphasized. In a home enviroment, the requirements for proper listening to 901's are too strict. Because the majority of the sound comes out of the back of these, you really need symetrical back walls for each of the speakers just to keep the sound balanced. Such common home furnishings as wall hangings, drapes, pictures & furnature can mess up that balance. And even though they may seem to be built like a tank, they are a short lived tank. The Bose 901's, 601's & the small 301's all have a "defect", as the drivers used in all of them have foam surrounds instead of longer lasting rubber. A friend has a pair of 901's on a shelf in his garage sitting doing nothing. All of the surrounds on all of the drivers have long since rotted away (but the cabinets still look good...). He is now using JBL's, & believes in the same comment someone once made at this forum - No highs, no lows, must be Bose!

[Audyossey]

...the series 901 speakers were still built like tanks.

Ah, but were they built like speakers?

...In a nightclub setting,...

Actually, my original response implying that their products may have improved was prompted by seeing and hearing a Bose system in a nightclub. It was not at all the typical Bose nightclub setup, but clearly a professional, permanent, custom installation with the subwoofer actually in the middle of the room near the mixing console. It's a club that generally has purely acoustic acts and the objective is a natural sound not breaking ear drums. That led me to think that Bose may actually be taking engineering seriously these days in at least some areas of its markets.

HB

[Geebster]

Ah, but were they built like speakers?

Close enough!! How can I put this delicately....errr... I can't... The Bose set-up I was referring to was in a ... Gentleman's club ... where you always go with a lot of dollar bills in your hands.

Sound Quality wasn't nearly as important as volume, to hide otherwise objectionable sounds that might be heard from such an environment. Be that as it may, it was one of the most pleasureable working situations I have ever been in!!

And it's where I met the GF, so all in all, it was a good thing.

GB