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Standard Weighting Curves
Included with Daqarta are several standard weighting curves, intended particularly for use with the Sound Level Meter option. In addition, there are RIAA standard curves for testing phono preamps, plus "pink" and "brown" tilt curves that can be used to make DaqMusiq darker or less bright.
If you know the parameters of a curve's transfer function (time constants, corner frequencies, or poles and zeros) and general shape, you can use the EQ_Curve macro mini-app to create other curve files. The EQ_Curve Help topic and its associated Example Curves and Parameters include instructions for reproducing most of the curves below (A-Weight through D-Weight, also RIAA and IEC-RIAA), plus their inverses.
A (A-Weight.CRV): One of the most common weighting curves and arguably one of the worst. It was an early attempt to reflect perceived loudness, but it is actually valid only for pure tones at relatively soft levels. Its response falls off steeply below 1 kHz and above 6 kHz, with a slight rise between those frequencies. This curve is usually available (along with C on even the most inexpensive sound level meters.
B (B-Weight.CRV): Flat between 400 and 4000 Hz, falling off steeply outside this range. It is no longer used but is included here for legacy work.
C (C-Weight.CRV): Flat between 100 and 4000 Hz, falling steeply above and below. This curve is used almost as much as A, sometimes in lieu of a true "Flat" response on inexpensive meters.
D (D-Weight.CRV): Special curve developed for measuring high-level aircraft noise, this curve has a large peak around 6 kHz to reflect human noise sensitivity.
Z (Z-Weight.CRV): (Not shown above.) The 'Z' is for 'Zero', formerly known as 'Flat' on some sound level meters. Here, however, the curve is flat only between 48 and 20000 Hz, and cuts off abruptly outside this range. This is useful with microphones (for example) that have poor response at very low and high frequencies, so their calibration files (.CAL or .FRD) have corresponding large negative dB entries. These cause the input spectrum to be boosted by the same amounts, in an attempt to create a perfectly flat response. The problem is that random acoustical or electrical noise in these frequency regions may be boosted by many dB, which in a quiet environment may rival or exceed the true signal. The Z-Weight curve simply ignores those regions, without the need to modify the basic .CAL or .FRD file.
Note that Z-Weight.CRV is intended for a 48000 Hz sample rate; use Z-Weight96.CRV for 96000 Hz, or modify as desired. Note also that the Sound Level Meter AC / DC Response button (in the AC position) effectively accomplishes the low-frequency rejection performed by Z-Weight, automatically adjusted for sample rate. However, it only applies to the Sound Level Meter SPL reading, not the Spectrum display, and the high frequency response is unaffected. Z-Weight works at both ends, and with the SPL readings as well as the spectrum display.
ITU-R 468 (ITU-R_468.CRV): Special curve based on research by the BBC to reflect the perceived loudness of all types of noise, including bursts. It also has a large peak around 6 kHz. It is intended to be used with an exotic "quasi-peak" response designed to give reduced sensitivity to narrow noise bursts, similar to human hearing.
ITU-R ARM (ITU-R_ARM.CRV): Identical to ITU-R 468 except shifted down by 5.6 dB so it crosses 0 dB at 2 kHz instead of 1 kHz. Instead of the special quasi-peak response, it uses a simple time constant. (ARM stands for Average Response Meter.)
These curves are useful for testing phono preamplifier equalization. See RIAA Phono Equalization Testing for more details. These files were generated by the EQ_Curve macro mini-app, which can be modified to generate other equalization curves.
RIAA.CRV: This is the Recording Industry Association of America standard phonograph playback equalization curve. It is essentially the frequency response that your phono preamp should have if it is driven by a signal with a flat response, such as white noise.
Inv_RIAA.CRV: This is the inverse of the above, and is useful for testing phono preamps. You measure the actual frequency response using, for example, the white noise or impulse response method. The response should have the exact shape of the RIAA curve, but it's hard to tell from looking at the raw spectrum. If you apply this inverse RIAA curve, however, you should get a flat horizontal line. Any deviations above or below it reflect errors in the equalization provided by the preamp.
IEC_RIAA.CRV: This is a modified RIAA playback equalization standard approved by the International Electrotechnical Commission in 1976. It rolls off frequencies below 20 Hz, but is essentially identical to the RIAA curve at frequencies above about 70 Hz. The image below shows the difference at low frequencies. Note that this is specified for playback equalization only, to be used with the standard RIAA recording equalization. It has seen little use, but some preamps may offer it as an option.
Inv_IEC_RIAA.CRV: The inverse of the above, used for testing preamps that support this.
Tilt-3dB.CRV: Tilts the spectrum downward at -3.01 dB/Octave to create pink noise from white. Long-term spectrum averages of normal music are said to have a pink spectrum. In general, it will make the music less bright. Note that you can achieve this same result by setting the Spectrum Curve Tilt to -3.01 dB/Oct. You can get its mirror using +3.01, which will compensate a pink response to make it appear flat.
Tilt-6dB.CRV: Tilts the spectrum downward at -6.02 dB/Octave, which is sometimes referred to as "brown" noise. In general, it will make the music darker by reducing highs. Note that you can achieve this same result by setting the Spectrum Curve Tilt to -6.02 dB/Oct. You can get its mirror using +6.02, which will differentiate a step Response to make it appear flat.
Otherwise, you can view the shape of any existing curve file by applying it to a signal with a flat spectrum. There are two basic ways to get a flat spectrum with the Generator: White noise or narrow pulses. For best results, view the spectrum of the Generator output directly in Daqarta (not a loop-back through an Input).
White (or Gaussian) noise has a flat spectrum when averaged over a long-enough time. You will probably find Exponential Spectrum Averaging to be easiest to deal with, with Frames set to 32 (default). The spectrum will still have a lot of tiny wiggles, but the curve shape will be obvious.
The spectrum of a single-sample pulse is perfectly flat, and doesn't require any averaging. Use a Pulse wave with the A Width set to 1 sample and the B Width set to zero. Set the Tone Frequency lower than the sample rate divided by 1024 (46.875 if the sample rate is 48000 Hz), so that you only get one pulse per trace. Note that this method is only perfect if everything is internal to Daqarta... if you try to create a real-world pulse that is then fed back to the Input, there may be distortions introduced along the way.
The spectrum must have Y-log (dB) active for Spectrum Curves to take effect. You will probably also want X-log active to see the curve the way it is customarily shown. You can use User Units and adjust Units/Volt to set the unweighted curve to 0 dB, so that the weighted relative response differences can be easily seen and measured with the cursors.
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