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RIAA Phono Equalization Testing
Phonograph records are recorded with a special equalization that reduces the amplitude of low frequencies and boosts highs. The constant velocity nature of the record cutting head means that, for a constant input amplitude, low frequencies produce a larger side-to-side excursion than higher frequencies. Wider groove spacing is thus needed to prevent "cutover" of the groove wall into the adjacent one, limiting the amount of program material that can be recorded on one record side. Reducing the amplitudes of low frequencies is thus a way to fit more program material onto a record.
Conversely, the vinyl recording medium has lots of high-frequency "surface noise" which is present regardless of the input signal. Boosting the amplitudes of high frequencies in the signal before recording makes them proportionally larger than the constant surface noise. Then when all high frequencies (signal and noise) are reduced in playback, the signal-to-noise ratio is improved.
The playback system must apply an exact mirror image of the recording equalization, precisely boosting the lows and reducing the highs to restore an overall flat frequency response.
There are several different strategies for testing the conformance of a preamp to the RIAA curve. Note that since the RIAA standard specifies dB relative to the response at 1 kHz, these tests do not require system calibration (which you would need to measure absolute levels).
One straightforward test method would be to measure the normal frequency response of the preamp, then compare it to the RIAA standard at multiple frequencies. This approach is complicated by the fact that the RIAA curve does not have a simple constant slope, making it very difficult to see deviations by eye.
For "Pass/Fail" production testing, Daqarta Professional users could use Spectrum Limits with the Max and Min limit curves set to define acceptable product limits. These could be as simple as the standard RIAA curve, shifted up and down by a given +/-dB limit.
Another approach would be to provide the preamp under test with an input signal that replicates the raw phonograph output of a recording of white (flat spectrum) noise... in other words, an inverse of the RIAA playback equalization. The RIAA equalization of the preamp under test should then result in a flat measured response spectrum. Deviations are then easy to spot by eye, and production tests using Spectrum Limits can use simpler horizontal Max and Min limits.
You can use the Band-Limited Noise option of the Daqarta Generator to create inverse RIAA noise. First, go to the Spectrum Curves dialog (in the Spectrum controls dialog) and click on one of the Weighting Curve File buttons, such as the top Weighting Curve0 File. Load the Inv_RIAA.CRV (Inverse RIAA Curve) file, one of the Standard Weighting Curves included with Daqarta.
Now go to the Generator controls dialog. Load the Default.GEN setup just to be sure you have a "clean slate". Click on Left Wave Controls, then click on Wave and select Band (under Noise). In the dialog that opens, set Rise Fc to 0 Hz, and Fall Fc to maximum. (Enter 1M to request 1 MHz, which will be automatically be limited to half of the current sample rate. You should be using the default 48000 Hz rate for this test, so the maximum will be 24000 Hz.)
Now toggle the Generator on and set the desired signal level with the F9 volume controls. In Spectrum mode you'll see a noisy but rising spectrum. Use Spectrum Averaging to smooth out the noise for a better view of the curve. (Use Exponential mode with a Frames Request of 32 for preliminary work, just to familiarize yourself with what's going on. For final testing use Linear with a Frames Request of 1024.)
To see what an ideal RIAA preamp response would be, even without any preamp under test, go back to the Spectrum Curves dialog and load RIAA.CRV into Weighting Curve1 File (or any other unused slot). Click on the LO button just below it to apply that curve to the current spectrum. You'll see it become a (slightly noisy) horizontal line.
However, note that there seems to be a small hump at around 50 Hz. Click on Spectrum X-log to get a more conventional log-log view, and you'll see this as a rise at the low end.
This is due to a limitation of the Noise Band approach to generating inverse RIAA noise; it is limited by the spectral resolution of the internal Finite Impulse Response (FIR) filter. You may be able to extend the useful low-frequency range by using a lower sample rate, but then you won't see the high-frequency range.
This is the preferred method. Unlike the above Inverse RIAA Noise approach, here you measure the ordinary frequency response using the white noise method. White noise does not have the low-frequency resolution problems of the Finite Impulse Response (FIR) filter used by Band-Limited Noise. (You can alternatively use the impulse response method, with some precautions.)
Applying perfect RIAA equalization to a flat input spectrum should yield a frequency response with the exact shape of the RIAA playback curve. As with the manual method discussed first, the fact that the RIAA curve is not flat would make it difficult to see or measure deviations from the standard.
The trick is to apply the Inv_RIAA.CRV Weighting Curve File to the measured spectrum, which will make it perfectly flat if the preamp perfectly follows the RIAA standard. Now, just as for the above Inverse RIAA Noise method, deviations are easy to spot by eye, and production tests using Spectrum Limits can use simple horizontal Max and Min limits.
Note that although there will be coarse resolution of low frequencies when viewing the entire spectrum, you can use Decimation to view the low frequencies with higher resolution, proportional to the Decimation Factor. However, you will also need to wait longer for the same number of frames from the Spectrum Averager.
If you want to simulate an ideal RIAA preamp, as discussed in the above Inverse RIAA Noise Method, you can't just apply an RIAA.CRV Spectrum Curve along with the Inv_RIAA.CRV... only one curve can be applied per channel. In fact, since one curve is just the negative of the other, if you could apply both they would simply cancel; you'd be left with the original flat white noise spectrum, as you might have expected anyway.
Actually, there is a way to do the above, if you have run Auto-Calibrate on your system to allow selection of the Wave Out User Line buttons in the User Units dialog. You'd click Left Wave Out and toggle Use Enabled Units and the Line Enable (marked 'LEFT Wave Out'), then click Load Speaker Cal File. The File Open dialog will appear, but it will only show .CAL or .FRD files. Nevertheless, you can directly enter RIAA.CRV under File Name, and it will be installed just like a calibration curve.
You'll then see a flat line (as expected) as long as User Units ALT+U) is active, and as long as Inv_RIAA.CRV has the LO button active in the Spectrum Curves dialog.
In 1976 the International Electrotechnical Commission (IEC) approved a modified RIAA playback standard, which has a strong reduction of frequencies below 20 Hz. Although this standard has not received much use, some preamps may offer it as an option.
To test a preamp for conformance to the IEC equalization, use the above White Noise Input With Inverse RIAA On Response method but substitute Inv_IEC_RIAA.CRV for Inv_RIAA.CRV. Note that since the difference with normal RIAA is only at the lowest frequencies, you will definitely need Decimation to test compliance.
You can use the same approaches as for RIAA to test any shape of equalization, such as NAB tape playback or something more exotic for special purposes. To create the needed Curve file you will need to know the characteristics of the target equalization curve.
For RIAA and IEC-RIAA playback, these take the form of 3 time constants (4 for IEC) that specify the corner frequencies or "break points" where the straight-line approximation of the curve would change direction. If your curve has the same general shape as RIAA (a flat low-frequency region, followed by a -6 dB/octave falling region, a flat mid-frequency region, then a further -6 dB/octave fall above that), or its inverse, you can easily use the EQ_Curve macro mini-app to create it. Just set it as for RIAA or Inverse RIAA (or IEC), but plug in new values for the time constants or corner frequencies.
EQ_Curve can actually handle an arbitrary number of transfer function parameters (time constants, corner frequencies, or "poles" and "zeros"), as long as you know the basic shape. See the EQ_Curve Help topic for a discussion.
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