linuxaudio.org

On Fri, Feb 17, 2012 at 12:32:01PM -0500, gene heskett wrote:
> [bolometers]

quoted text
> 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]

quoted text
> 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.
_______________________________________________

Linux-audio-dev mailing list

Linux-audio-dev@lists.linuxaudio.org

http://lists.linuxaudio.org/listinfo/linux-audio-dev