Electronics > Beginners
Audio Conventions - Levels and Meters
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Harb:

--- Quote from: dave_k on May 27, 2018, 11:33:14 pm ---
--- Quote from: Harb on May 27, 2018, 09:29:01 am ---You will do very well to nail down a standard in Audio........ There are so many "Standards" depending on who you talk to.....

I am in the TV industry, so the only one we are interested in is +4dBm (600 Ohms) after all is said and done....it used to be +8 a few years ago, but +4 is now the accepted level....
Having said that, with the acceptance of HD things have become different again and measuring much more complex.

--- End quote ---

In my 20 or so years in broadcast, both TV and radio, I haven't seen +8dBm used as a reference level (except for the old Fox FM studios in Melbourne). Going further I haven't even seen +4dBm used as a reference very much.

Most professional audio equipment nowadays feature low source impedance and high input impedance, so +4dBu is almost universally used.

--- End quote ---

I've been in a while too Dave....to long lol 26 years this year, I still like it, but miss the days of long haul microwave links and getting to sit on a hill somewhere and relaxing......again this just shows how varied the so called standards are.....I have level meters here that were built by the broadcaster/s that were calibrated for +8dBm and later changed to +4dBm as they started to realise Valve gear was changing over to solid state and overcoming noise figures was less of a drama......we still are requested to feed use a +4dBm 1K tone into the remaining analogue feeds .
ed_reardon:
My gratitude to all of you, there's some great information here for me to go and research!

Thanks again,
Ed
IanMacdonald:
Standard line level is 0.775 volt RMS, which is also 1v peak, and 600R source impedance. Sounds like that ought to be simple, but the problems mostly arise from the European DIN standard which specifies a high source impedance such that the actual voltage depends a lot on the load.

Metering is either VU or PPM.  VU meters respond to average voltage (a simple half wave unfiltered rectifier) and 0dB is the correct recording level for the loudest passage. PPMs respond to peak voltage (both in the sense of Vp for the waveform and the max encountered over a time interval of a few hundred ms)  and are scaled 0-5. The correct recording level is typically 3. 
dave_k:
This is a good reference chart when it comes to matching audio levels across different areas of the globe.
ed_reardon:
Cheers again all.

As I want to use this as a mixer-recorder (it's essentially going to be a small 4-channel mixing desk with a built in MP3 codec and SD card) unless I've totally misunderstood I can set 0dB at the typical V(pp) of the line-input in to the encoder IC:

Typical Line Input = 2500mVpp= 1250mV half-wave peak.

Using an LM3915 with the half-wave peak detector and 1250mV full scale value should yield a usable meter, there's a bit of head-room with the numbers as harmonic distortion doesn't become apparent until 2800mVpp by the spec' sheet.

By my reckoning this should be okay for setting channel gain, this doesn't need to be broadcast standard driven, so as Dave says, you can always make up your own dB scales!

Hope this makes sense,  audio really isn't something I've thought much about.




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