Can I efficiently monitor sound card line in with a vu meter?

Question

I'm trying to monitor some audio input to my desktop pc with a vu meter. I've connected to the line in, and it's just a bog standard on-the-motherboard sound card. I'm having a problem understanding the various signal levels. I haven't actually bought the meter yet as I need two antique ones at some cost, so at this stage I'm looking at the feasibility and risks involved.

From Wiki, consumer grade kit is designed for a nominal -10dB line level. Recording via Audacity, it appears that my sound card line input actually clips at -6dB so there is very little headroom. The typical analogue (it must be analogue) vu meter is calibrated -20dB to +3dB. The -10 mark is only approximately 1/7th of the way up the scale. To avoid the risk of clipping, I'd need to run at a level less than – 10dB. That doesn't seem a very efficient use of the meter real estate. I'm assuming that vu metering is for absolute readings and not arbitrary scales. I only have rudimentary knowledge of this area, but have I miss understood something?

Answer 1

Yes, Paul, you are missing something basic: The fact that dB is not an absolute measurement, but a relative one. You need to know what your VU meter is relative to.

Could your VU meter is reading dBm instead of dB? dBm is a measurement relative to 1 milliwatt, with 0 dB being a milliwatt. If not, then the VU meter is relative to something else, specific to the application it was designed for.

As an example, the VU meters on a power-amplifier are usually relative to the maximum continuous power, with 0 dB being equal to 100 watts for a 100 watt amp. So most of the scale is negative dB, with the positive dB being in the clipping region (an aside: it might be an issue different than clipping that sets the maximum continuous power for the amp). With +3 dB as your VU meter's maximum, it seems like it is a relative reading, and not dBm.

In audio, most dB measurements are relative to full-scale, so you are almost always dealing with negative values.

If you can determine what the VU meter is measuring relative to, and also determine what Audacity's dB scale is relative to (possibly 1 volt p-p), then you can determine a constant value to add/subtract from the VU meter's reading to correspond to Audacity's dB reading.

Answer 2

By definition, your analog-to-digital input (or anybody's input) clips at 0dBFS (the decibel ratio relative to Full Scale).

Likely the most accurate way to meter an input like this is with a software meter. That means the meter is showing the ACTUAL level out of the Analog-to-Digital converter. And THAT is what matters most. Any other kind of metering scheme will have to be calibrated against the internal A-D converter, anyway. Might as well skip the extra step and ambiguity.

Answer 3

Although VU meters measure dB, they measure dB relative to some standard level, normally 1mw into 600 ohms, or 0.775Vrms, so in practice they are an absolute measurement. You still need to calibrate this level against your soundcard though.

However, VU meters are inappropriate for digital audio measurements - they are relatively slow response, effectively indicating average energy rather than peak levels.

With digital signals, it's important to measure peak levels, therefore PPM (Peak Program Meters) are more appropriate. These were adopted in broadcasting because the AM modulation process clips (on negative peaks) as hard as a digital system at full scale, therefore detecting short term peaks was important.

In the UK, broadcast engineers were trained to "peak to 6" on the scale of 1 to 7. The divisions on the PPM (IEC 60268-10 type II) are 4dB apart, with 0dB/1mw/600 ohms corresponding to "4" on the scale, so "6" represents +8 dBm.

This is still quasi-peak, with a response time of about 1ms, so it under-reports fast peaks. So, when digital recordings became common, full scale was set to +18dB, providing 10dB of headroom above "6" on the PPM.

Answer 4

Can you post a link to the unit you´re looking at?

In any case, analogue VU meters are essentially current driven, so you can change the 0dB level for your setup by changing the shunt resistor on the meter.