а на Штудер Дайджест в это время...
Subject: Re: [Studer] a dB is a dB is a dB? (not)
Hi everyone,
At the risk of extending a slightly off-topic subject too far, but as it is of great relevance to
anyone wishing to interface Studer recorders correctly (not much use without connecting them to
other electronics!) and therefore getting the best performance from them, here goes...
Firstly thanks both to yourself and Richard for the informative posts! I'd like to clarify just
the first point
On 23.05.08 12`46 am, Stephen Anderson wrote:
> The original dBu, was specified as 1 mW into 600 Ohms, which calculates out to approx. 775 VRMS.
Obviously it is impedance specific.
...you are correct, of course, that the definition of the new (at the time) dBu from the existing
dBm used an assumption of 600 ohms, being a broadly accepted 'standard' in common use - adopted
from the pre-existing telephony industry I believe.
However, Richard's statement was that the _dBm_ was not impedance specific, as is commonly
misconstrued since it is often described as "0.775v into 600 ohms" for people using brought up on
voltage-designed amp circuitry rather than power transmission lines, when strictly it is as
Richard
stated, "1mW". He made no such claim for the _dBu_.
> dBV ref. 0=.775, while commonly used, has been a recent (70's) attempt to quantify the same
voltage with no impedance spec. Thus, many manufacturers state that their gear will output +4
dBV, or
1.228 VRMS, with no regard to the bridging impedance their output sees.
[Correction: I think you meant dBu in the above paragraph, and not dBV.]
...a good point which is important not to forget, despite most (all?) modern audio equipment
having source impedances typically at least 100 times lower than input impedances making it
largely
irrelevant in practical situations.
Richard wrote:
> 0 dBV = 1.0 V (I've seen this written with both lower- and upper-case "V" -- this is used in
consumer equipment
Traditionally lower-case letters are used for ac quantities, and upper-case for DC, so IMHO it
should be really be written as dBv rather than dBV - I wonder if it's referred to as the latter
in
any authoritative documentation, such as AES papers???
> dBV ref: 1V was the old Tascam standard, championed by Dick Rosmini, with the standard as -10dBV
ref. 1V, or .316 VRMS, which, IIRC, Tascam rounded off to .3 VRMS.
...reflecting practical usage referring only to voltage, so why not reference to an existing unit
of definition? (being in the business of manufacturing voltage-matched gear) Standards
organisations, us???
> FWIW, I always thought that depressed level was silly, the same voltage rails, the same ICs,
just a lower operating level, and RCA jack. Not a really good idea.
Well, being unbalanced the drive capability is already reduced by 6dB ...seems odd to have adopted
the -10dB lower reference voltage level for no reason. I think they like round decimal numbers! I
imagine it would have been for economic reasons. A trade-off of noise vs. distortion? Cheaper
parts performing better when operating well away from the voltage rails perhaps? Less demands on
PSU
circuitry ...hmm - thinking aloud here.
> And many broadcast facilities, at least here in L.A., used to use +8dB ref. 0=.775 as their
standard, although today, I'm not sure. I do know that back in the '90s, NBC Burbank was still
using
150 Ohm transformers for long tie lines around the facility.
>
> Steve
Thanks again for these comments - I was unaware of this piece of history.
Standards, don't you love 'em? The great thing is we have so many to choose from
More clarification than confusion I hope!
Steve