Subject:78 record EQ

Posted by: AFSDMS

Date:1/16/2006 1:49:19 PM

I have a turntable with cartridge and a couple different stylii for 78 rpm (OK 76-84 rpm) records. I also have a very high quality USB 2.0 digital interface (M-Audio FastTrack Pro 96 kHz/24-bit) and would like to try recording FLAT using the high quality mike preamp then using SF7 EQ to correct for the various standards. I've seen pages on the web with a couple EQ setting schemes but they all relate to using the octave equalizers.

With the features of SF I'm wondering if it would be better to use the Parametric EQ and cascade two low frequency shelves (or two high frequency shelves) or should I set up an envelope in the Graphic EQ. The problem with this latter option is that you have to drag the points for the graphic curve and the accuracy is mediocre.

I've also had a hard time finding clear and non-conflicting descriptions of the turnover and cutoff frequencies and figuring out how to convert them into EQ curves. (However, there are great resources that list the EQ used on lots of records over the years and the likely range of disc RPMs.)

Any suggestions and experiences regarding EQ greatly appreciated.

Subject:RE: 78 record EQ

Reply by: rraud

Date:1/16/2006 4:25:46 PM

Aside from using a good "PHONO" preamp.
A lot was discussed about a year ago or so.. Do a search on this forum for, vinyl EQ, phono, records, and other associated keywords.

Subject:RE: 78 record EQ

Reply by: AFSDMS

Date:1/16/2006 5:52:59 PM

I had searched prior to making my post. I also searched again trying more keywords. My specific question has nothing to do with the basic postings I was able to find. All I could find relate to noise reduction and click/pop removal in discs with cracks. (My suggestion is to take up skeet shooting :-)

I have a great phono preamp but that has nothing to do with the question. The point was to most effectively utilize the capabilities of SF, capture a broad range of discs and be able to later process/EQ them optimally.

Anybody else out there lurking who has time to read my request and offer a suggestion?

TIA

Subject:RE: 78 record EQ

Reply by: Geoff_Wood

Date:1/19/2006 4:12:50 PM

Looked like you were planning to do the phono RIAA EQ in software, which is a totally flawed approach. But try it anyway.

geoff

Subject:RE: 78 record EQ

Reply by: Chienworks

Date:1/20/2006 4:00:53 AM

Since you have a "great" phono preamp, use that. It has been constructed to accomplish exactly what you want to do, and it's built in EQ is surely better than any software could do.

Subject:RE: 78 record EQ

Reply by: mpd

Date:1/20/2006 11:30:03 AM

I don't think you can use a simple parametric EQ to simulate an RIAA curve, or any of the other curves. I belive most of them are simple second-order filters, but they don't fall in any of the standard filter types.

If you are serious about trying this, then you need to find the s-domain transfer functions, use one of the various methods to convert this to a z-domain representation, and find a plugin that can implement an arbitrary biquad. Someone on comp.dsp or the music-dsp mailing list may be able to help.

Subject:RE: 78 record EQ

Reply by: Geoff_Wood

Date:1/23/2006 1:45:34 PM

.... which may implement the RIAA (or equivalent) eq, but will leave the fundamental problem of totally compromised headroom at one frequency extreme, and potential noise at the other.

Just stick to the phono preamp eq for starters, and tweak whatever *variation* from that with eq. If you cannot find the actual eq spec used on a particular 78, then all you can do is EQ to your own satisfaction.

geoff

Subject:RE: 78 record EQ

Reply by: AFSDMS

Date:2/5/2006 12:29:28 PM

I guess I assumed too much. Yes I have a great phono pre-amp, but it is modern and supports RIAA. 78s pre-date the RIAA implementation. Each record label had their own choices of turnover and rolloff frequency combinations. Even then there was variation within a label depending upon year and even the record producer. You can find lots of data on sites about the curves, yes simple filters, they used from label to label and over time.

Since my first question I've done some experiments. Starting with a very flat (no RIAA) preamp I have captured the raw uncorrected audio from a disk. I then used the Graphic Equalizer's Envelope mode. This allowed me to set points at the exact frequencies necessary to replicate, in inverse fashion, the EQ applied to the recording when it was cut.

I haven't seen any issues with noise even though the gain difference at the two extremes of the EQ is 42 dB. Keep in mind that the source material is 78 rpm recordings and I have something like 90+ dB s/n in the front end of the system. It sounds absurd, but I'm using 96 kHz/24-bit to get the maximum headroom.

I'm also thinking, for the sake of humoring an audio engineer friend's suggestion, that I reduce the EQ to 50% of what is needed and apply it twice to the file. The second pass is after the audio has been reversed. Verne once told me that the EQ impacts the phase relationship of the audio and by doing it this way you would cancel much of the phase shift. I was relying on his expertise with digital audio restoration and may just try it in his memory.

I'll keep experimenting but capturing the un-EQed audio does have the advantage of not having to undo an improper selection/guess at the first go around.

Subject:RE: 78 record EQ

Reply by: Chienworks

Date:2/5/2006 2:08:13 PM

"The second pass is after the audio has been reversed. Verne once told me that the EQ impacts the phase relationship of the audio and by doing it this way you would cancel much of the phase shift."

That's a new one for me. Maybe he's right, but i can see two objections. I would think that the phase distortion caused by EQ would be more like smudging. Doing it again in reverse merely smudges the already smudged signal. I think the end result would be doubling the problem rather than correcting it. Secondly, each time you run a process on a file you lose some of the original fidelity. If you're that concerned with preserving quality then the best method is to do as few processes as possible.

Subject:RE: 78 record EQ

Reply by: mpd

Date:2/6/2006 6:53:16 AM

Running a data set (in this case, the audio file) forwards and backwards through a filter is a standard trick for undoing the phase shift in a non-linear phase filter. The math is a bit much to get into here, but I can provide some references for those with access to decent libraries.

Subject:RE: 78 record EQ

Reply by: AFSDMS

Date:2/11/2006 10:13:23 AM

So MPD, do you think it might be of value or would it just make things worse? Unfortunately Verne is no longer around to consult :-(

Wayne