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Old 9th March 2004, 11:47 PM   #11
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by Havoc
I use a simple flat input stage and so to the pc. Correction will be done in soft and then to CD. After a bit of noise and click reduction. But this project is on hold for the moment because of pc trouble.
Eeek! Bear in mind that even 78 equalisation can encompass 30dB of dynamic range from the level at low frequencies compared to extreme HF. In other words, you have just raised the noise floor at LF by 30dB. If you then consider that 78s have lots of pops and clicks, you will have to lower the level fed to the soundcard to ensure that they don't cause digital overload. Another 12dB would probably be advisable. Simply doing arithmetic on numbers and rounding them back to the same number of significant digits causes quantisation errors that must be dithered, knocking off another 3dB.

Applying disc equalisation digitally uses up so much dynamic range that there may not be much left for the music! Inconvenient though it is, analogue will be better than digits.

No, I don't have 78s, but I've looked into the electronic problem for people who do.
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Old 10th March 2004, 10:34 AM   #12
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It is not that bad.

First of all, I use a 24bit conversion. So low levels are not a problem. Next, with a bit of care you can go up to -3dB at the input without any problem. If the pop is so big to possibly clip the input, you will need to cut it out anyway, so you loose nothing. Finally all subsequent work is done in 32bit floating point, so this introduces no clipping and makes sure rounding is no real issue until final conversion to 16bit for CD.

While analog EQ may be indeed a bit better, the problem is that there are about 10-12 curves. I don't like to put loads of switches in such circuits, and at the same time like to keep them compact. So it would mean 10-12 preamps. It can also be a hit and miss finding the best EQ for a given record. Certainly if some obscure label and you cannot date it for certain. So doing this digitaly is a lot easier.
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Old 10th March 2004, 11:55 AM   #13
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All good points, especially the 32 bit and rounding issue.

Your point about the lack of problem with large clicks being clipped is interesting. I would expect there to be a buffer between the input sockets and the A/D. If they are well matched, the buffer may not have any significant headroom above the A/D. Clipping the buffer might cause latch-up, prolonging the length of the click. It might be worth investigating.

When looking for curves, I was looking mainly for pre-RIAA LP equalisation. I found that to cover all possibilities, a 12 position switch was needed for bass eq and another for treble. If you reckon you only need 10-12 settings for 78s, you are getting off lightly. Still, it makes digits attractive.
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Old 10th March 2004, 06:44 PM   #14
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I would expect there to be a buffer between the input sockets and the A/D. If they are well matched, the buffer may not have any significant headroom above the A/D. Clipping the buffer might cause latch-up, prolonging the length of the click. It might be worth investigating.
Good point. I do now the preamp can output to +/-12V before coming into trouble. So it would be inside the AD. Could not really find any specs on that. But I haven't yet seen it, but this is no garantee. The Ad has a setting for inputs upto 0dBFS@+19dBV, but as the gain structure is unknown, again it tells you nothing.

Now if you have a click, the application itself looks at the spectrum before and after and fill in according to that. So the click itself is not used for this. If the latch-up would occur, it would simply mean you have a slightly larger click to fill. As clicks can be several ms long, I don't think it would be a problem. But it is certainly something that needs to be checked before you start processing.

The digital route is attractive, but as I have no analog alternative, it is hard to tell if it really is an alternative. I guess it depends on what you want to achieve. For me it is nothing more that being able to listen to those discs. And I don't have enough of them around to justify an analog preamp.
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