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Now you can use your laptop PC to make high-quality stereo audio recordings with professional standard balanced microphones. This interface unit lets you make recordings at sampling rates up to 48 kilosamples/second and provides high-quality stereo analog line outputs for playback or monitoring. There's also an S/PDIF digital audio input for recording and an S/PDIF digital audio output for playback.

How accurate is GPS speed measurement?

I have noticed that in the work car, to drive at 110km/h by the GPS unit (a well-known brand) I need to drive at close to 125km/h by the vehicle’s speedometer. While I realise that the car speedometer is probably reading high, I am aware that the GPS unit approximates the Earth’s surface and therefore potentially also is in error.

Using the other traffic as a guide, I expect that the GPS unit is reading fairly accurately but given the expense should I get it wrong, I was wondering how much of an error will the GPS unit produce – is it close enough that the “grace factor” applied by the speed camera units cover the potential error of the unit? (T. M., Paradise, SA).

• This topic of large vehicle speedometer error has been discussed in the past and is a consequence of the Australian Design Rules which specify that the speedo must be optimistic (ie, give a high reading) while the odo­meter is supposed to be highly accurate.

Your experience is typical. While there is bound to be a small GPS speed measurement error you can expect it to be less than 1%. By comparison, vehicle speedos are typically 8-9% high in their readings at speeds of 100km/h or more. Be warned though, vehicle speedometers are usually fairly accurate between 40km/h and 70km/h. So don’t go above the speedo indication in areas with these speed limits otherwise you risk being booked.

Preamplifier For A Flexible Piezo Film Sensor

I’d like to use the Pre-Champ (SILICON CHIP, July 1994) to amplify the signal from a flexible piezo film sensor – see http://www.meas-spec.com/product/t_product.aspx?id=2478#

However, what would I need to do to match the Pre-Champ to such a high-impedance load – possibly two, either in series or in parallel? I’m not too sure at this stage if it would be worse for bass response.

Is there a frequency response chart available for the Pre-Champ preamplifier? (P. S., Lane Cove, NSW).

• The data sheet indicates that the recommended load impedance is 10MΩ, which you would need to get maximum bass response from the sensor. With this in mind, the Pre-Champ is never going to the job. Besides which, its -3dB bass roll-off is at 72Hz – not good enough, in all probability.

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