(The following is the current version of the anti-aliasing notice. It has not changed substantially since InfoSignal News 7.)

Anti-aliasing plays an important part in insuring reliable and accurate frequency measurement in speech signals. We would like to suggest with some urgency that you to read the relevant passages in the Manual (pp. 83 and 176). Well before you start on your research project, please be sure to conduct the requisite anti-aliasing testing for your specific input device, and that at your specific sampling frequency.

At the present, there are a fair number of sound input devices on the market, so it is quite impossible to provide accurate and up-to-date information for all devices in use. However, to give some indication of the anti-aliasing status of the most common in- and output devices, a short survey is provided below.

The following information is accurate to the best of our knowledge. However, no matter what is said here, you must establish to your own satisfaction that your anti-aliasing is satisfactory for your purposes.

Why You Need Anti-Aliasing Filtering

Anti-aliasing filtering reduces or eliminates frequency components greater than half the sampling frequency (the Nyquist frequency). If you sample at 16 kHz, you should anti-alias at about 8 kHz. The actual cutoff frequency should be set at 40% of 16 kHz, or 6.4 kHz, because filter cutoffs are not so much like "steps", but much more like "hillsides". That way you're sure to have good anti-aliasing for frequencies starting with the Nyquist frequency.

Anti-aliasing filtering is needed prior to sampling because without filtering, frequency components below half the sampling frequency will be contaminated by frequency components above the cutoff point. If you don't anti-alias, frequency information will be inaccurate. You will probably be able to recognize the frequency pattern, but the amplitudes associated with specific frequency responses will be either too great or too small. Also, frequency components may appear at frequencies where in fact, there was no frequency present. This phenomenon is much like the "ghost images" you may get on TV in unfavorable reception areas. Further, frequency peaks may shift in aliased conditions.

You must anti-alias the signal prior to sampling. You cannot "repair the damage" by software filtering an un-anti-aliased signal input after sampling.

You can record at any sampling frequency if you can assure anti-aliasing filtering for it. The final choice of sampling frequency is related to the highest sound frequency to be analyzed and to available disk space, not to anti-aliasing.

Anti-aliasing filtering is also needed subsequent to D/A conversion (sound reproduction), because otherwise, the signal may sound harsh or "metallic". In this case, the filtering more correctly called "anti-imaging filtering". This is because the D/A conversion process, or the upsampling process prior to D/A conversion, may introduce spurious high frequency elements that are not related to the signal.

The Apple Built-in Mic

InfoSignal (the creator of Signalyze) did not recommend the Apple built-in microphone for research purposes; Linguist Plus upholds this lack of recommendation. However, with some caution (think of it as "informed consent"!), the Apple attenuated input from tape recordings can be used.

Here are the reasons for not recommending the Apple mike:

1. The microphone operates with automatic gain control (AGC). This is good for vocal note taking--which is the prime purpose intended for this mike. However longitudinal amplitude variation cannot be encoded accurately when AGC is on. While Signalyze does its best to turn off the AGC, it appears that the system software turns it back on at its first opportunity.

2. The AGC is set to make sound pickup possible even at relatively great distances from the mouth. While this enhances audibility, it renders the mike input very noisy for signal analysis. It also distorts the sound when the microphone is held at closer range.

3. It is an 8-bit device. High-frequency information is inadequately encoded by 8 bits, particularly in weaker passages of the signal.

4. Apple provides no information on whether the microphone incorporates anti-aliasing. The literature we've seen seems to say that this is not handled in the same manner on all Macs that have sound input. Our best guess, based on comparing unfiltered with filtered signals, is that there is anti-aliasing present, at least on some machines. Make your own comparisons with peak-to-peak measures (see Manual, p. 84).

If you must record with the Apple built-in mike, try holding it about 6 inches away from the mouth while speaking and speak clearly. That's the only we managed to get a decent input signal in our own experiments.

Apple Output

Different Macintoshes incorporate different hardware to reproduce sound. Old system Mac synthesizer (MacPlus, SE) apparently do not incorporate anti-aliasing. This may account for the "harsh" sound rendering on these older machines. The new system Mac synthesizer (all recent machines) apparently does incorporate anti-aliasing.

MacRecorder

A simple capacitor-based filter is incorporated into MacRecorder, providing moderate anti-aliasing. For many research purposes, this seems adequate.

It is possible to record into the aux input of MacRecorder. This appears to provide reasonably clean, anti-aliased input.

Again, MacRecorder is an 8-bit device and therefore poses severe limitations on the encoding of high frequency information. Still for 8-bit sound recording, MacRecorder is the current input device of choice.

Mac Adios Input and Output

MODEL 411, MacADIOS II: These devices are typically sold with a separate amplification unit that incorporates anti-aliasing filtering for both input and output. Higher-end boards are also sometimes sold with a plug-in anti-aliasing filter. One Signalyze user reports a 5 kHz low pass cutoff for the AFS daughter. Anti-aliasing on the MacADIOS devices thus appears satisfactory if you can "live with" a 5 kHz cutoff. Users who do not own this amplification-filtering unit should minimally place some other filtering unit between their amplifier and MacADIOS input. Output filtering is desirable, but less essential.

AudioMedia and Sound Accelerator Boards

AudioMedia and Sound Accelerator boards have an on-board hardware filter set at 22 kHz. On these devices, hardware anti-aliasing is automatic for 44.1 kHz.

Also, anti-aliasing at any sampling frequency is available for Signalyze direct I/O in versions 2.10 and higher. On earlier versions of Signalyze, anti-aliasing is not warranted at any other sampling frequency than 44.1 kHz.