How does a "Software RIAA preamp" work ?

2006-04-18 10:51:40

I'm currently working on a little recording application and i've seen some DJ Software which offers "Software RIAA preamp".
With this feature it should be possible to use record player signal without a preamp 
What it does ? Just amplify the signal ?


regards,
sick

How does a "Software RIAA preamp" work ?

Reply #1 – 2006-04-18 11:12:42

I guess it just applies the RIAA equalization curve to the recording, which needs to be done at some point either by hardware or software for the vinyl to sound right. If it's already been done once in the recording chain, you don't want to apply it again.

How does a "Software RIAA preamp" work ?

Reply #2 – 2006-04-18 11:38:18

Any audio editor can do it. If you use a preamp without equalization, then the inverse RIAA Eq can be applied in software; it just means applying the proper gain and cut at the proper frequencies. The numbers can be found many places since RIAA is a universal standard for LPs.

I've never played with doing this. It has supporters and distractors. One of the main points of the distractors is that without the large high frequency cut applied in hardware, it is possible to overdrive many preamps, resulting in a lot of HF distortion.

None of the pro arguments I read was the least convincing to me. They mainly run along the line of 'with the increased high frequencies it is easier for declicking software to do its job." No evidence to support such a claim that I can see. Not needed anyway; good software does fine with proper recordings.

If you try to record with no preamp, you will get a pretty crappy recording that will never amount to anything worthwhile. You cannot make up all that gain in software to any advantage; you will lose too much resolution, even from LPs. You cannot load the cartridge properly.

A line level preamp isn't going to cut it, there isn't anywhere near enough gain and it does not have anywhere near the proper input impedance. To do this you need a genuine phono preamp that is just missing the EQ. Why bother? Someone just wants to sell you a useles gizmo. Maybe a pink rabbit's foot will give your recordings that special edge you need.

How does a "Software RIAA preamp" work ?

Reply #3 – 2006-04-18 16:47:58

One of the bigger pros I've heard about software preamps is that you can use an arbitrarily accurate FIR filter to get an extremely good conformance to the RIAA response. But I don't think that's even an issue nowadays with most hardware preamps. My $40 jobbie does +-0.5db at all frequencies. It's really not that big a deal.

How does a "Software RIAA preamp" work ?

Reply #4 – 2006-04-18 18:18:16

I have a special case.  I own 78 rpm records but lack a 78rpm turntable.  What I do is play the records at 45 rpm using a high impedance mike preamp to feed the sound card.  Then I use Audacity to correct the 78/45 speed diff, and Audacity again to apply RIAA eq.  It works just fine.  Just be carefull about levels through out each step.  It's easy to create distortions or digital clipping.

How does a "Software RIAA preamp" work ?

Reply #5 – 2006-04-19 07:30:34

It is easy to get a good speed match in software this way. This can be especially useful with "78s" which were actually created at many different speeds. I don't recall exact figures, but I believe the range used was something like 65 rpm to 85 rpm. There are only a few turntables on the market that can deal with them all through a variable speed adjustment.

Most, but not all, 78s were cut with some kind of frequency compensation EQ. None of them used the RIAA curve, however. Most of the curves used are pretty far from the LP RIAA EQ. There are a number of web sites that list figures for a good many labels, and the years they used which particular compensation. Since I don't do many 78s, I don't have any of the addresses on hand.

The potential problems with recording 78s without a adequate phono preamp are similar to those for LPs, but perhaps not as generally severe. The cartridge is probably not going to be properly loaded by a microphone preamp. This is supposed to make a significant difference in the quality of the signal one gets from it, but not having tried myself, I can only report what I've read.

The software EQ approach is better than nothing, and the only half way affordable way for many people to deal with 78s. Good 78 preamps, with a wide variety of selectable EQs, tend to be rather expensive, but they probably ultimately give better results. One would not have such a problem of getting a reasonable transfer into the computer when using one.

The RIAA compensation for LPs has quite significant gain on the bass and reduction on the treble. Recording flat means that the high frequencies are at a much higher level that the lower frequencies. Adjusting input level to avoid clipping the highs means that resolution is definitely reduced at the frequencies for most music fundamentals, both voice and instruments. I believe most 78 curves are simpler that the RIAA one, but it still might be better to record with a standard phono preamp, undo the RIAA compensation, then apply the proper 78 EQ.

How does a "Software RIAA preamp" work ?

Reply #6 – 2006-04-20 18:40:05

The problem I have with this is the amount of dynamic range you have to give up in order to make this work.


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