[78-L] Groove spacing - groove width

[J. E. Knox]

Greetings from FixitLand!

Ron Roscoe wrote:

> I think you mean bass CUT on recording.

Yes, that's correct. Got a lot of things going on at once. Brain flatulence.

> I'm pretty sure that the
> 250Hz-300Hz turnover stayed for a long time.  I would like to know where you
> get the 500 Hz value; perhaps I missed it somehow!

Spectrum analysis of many records of the era over the last decade or so. Looking at a spectrum analysis tells you much about what's on that disc. You can see the point at which the bass rolloff occurs (not to mention whether or not there is treble boost). Further, RCA literature refers to "200 cps and 500 cps" records. And, you may find the 1940 RCA Recording Department manual to be an interesting read. Everything you never wanted to know about how Victor and Bluebird records were made during that time. Some of the blueprints go back a few years prior. Download the manual here:

http://rayrayburn.com/rca/RCA_manual.pdf

> But, I honestly think
> that the 500 Hz turnover would have provided a seriously noticeable
> low-frequency loss in playback without any compensating playback bass boost.
> Do you think that the bass boost only started to occur when crystal
> cartridges became widely used?  A crystal has a natural playback
> characteristic that includes some bass boost as well as treble cut.  I think
> that by the time crystals became widely used the recordings were also made
> with treble boost to help cut the high-frequency record noise.

I've found that phono amps using crystal and ceramic carts have NO equalization added — as you note, the piezo element "does the work." Phono EQ seems needed primarily for magnetic pickups. The treble boost appears on commercial records just before World War II. A good example would be the first Capitol records, the sleeves of which trumpet an "All-New Method of Recording." Spectrum analysis reveals just that — 500 Hz bass turnover as usual, but a high-frequency boost. Those records sound about right using an NAB playback curve.

> Gary Galo has written a wonderful account of record equalization:  "Disc
> Recording Equalization
> Demystified By Gary A. Galo"  I don't have the magazine title from which the
> article came but you can download the article here:
> www.novotone.eu/_site/projets/Projet06/Doc01.pdf
> Gary, love to hear what you have to say about this!

That article quotes a 30 October 1935 letter from Victor, describing the recording characteristic. I've seen that letter too. Spectrum analyses of a number of records from that timeframe appear to be substantially in agreement. I will read the article in detail as time permits. Thanks for the link!

> Yes, I have also viewed the 10-51 schematic.  I have a friend who has a huge
> collection of pre-1930 Victor consoles with either changers or single play
> turntables and he claims NONE of them have compensating bass boost for
> playback.  That's why I wonder when the playback equalization might have
> begun.
> Fascinating topic!

Even with the rolled-off bass recorded on the record, I'm sure that the response was much improved over acoustically-recorded records of only a few years prior. It would have been a marvel.

Take care,


—
J. E. Knox "The Victor Freak"


-- 
*Celebrating 30 years of service.*