[LAD] [ot] rme fireface: weird balanced output measurements

[Fons Adriaensen]

On Fri, Feb 17, 2012 at 12:32:01PM -0500, gene heskett wrote:
 
> [bolometers]
> Either of those methods costs 500-5000 USD to accomplish.

The average house & garden multimeter is indeed completely useless
for measuring anything audio. OTOH

* There are quite good handheld audio RMS meters which don't 
  cost a fortune (but they are in the higher price range).
  They use analog integrated circuits which can be quite 
  accurate - at least for normal audio use. They are not
  laboratory standards of course. 

* Any pro-quality audio card, once calibrated against a known
  signal and combined with some simple software will make a
  near-perfect RMS meter *for the audio band* and as long as
  you don't drive it into clipping. 
 
> [cable impedance]
> Why?  The common two wire & foil shielded audio cable, used in broadcast 
> and studio facilities in miles per studio quantities, actually has an 
> impedance in the 60 ohm area!  Feed it with a 600 ohm source and 300 feet 
> of cable later its rolled off like a Ma Bell telephone circuit.  Your audio 
> DA's, to drive that, need to source terminate at 30 ohms per wire, from a 
> very low impedance amplifier.

The concept of cable impedance makes sense only if the lenght
becomes a non-trivial fraction of wavelength. For audio that
means that for anything shorter than a few hundred meters it's
only capacitance that matters. And yes you need hefty line 
drivers and low output impedance to push 20 kHz, +20 dBu on
a long line. Which is one of the reasons why real pro quality
analog audio remains expensive.

Ciao,

-- 
FA

Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.