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Old 19th Apr 2018, 5:46 am   #1
Synchrodyne
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Default dBV, -10dV into 10k, etc.

When did dBV (where 0 dBV = 1 volt) come into use as a unit for measuring signal levels in domestic audio system interconnections?

And concomitantly, when did the -10 dBV level, into 10k impedance, become a standard, or at least a de facto standard, for domestic audio unbalanced “line” interconnections?

Is there a formal standard covering the -10 dBV, 10k numbers?

I have assumed that the -10dBV level (316 mV) refers to average or typical programme, which if one uses the “old” number of 30% modulation as a representative average, implies peaks of 1 V for most sources. But I have never found any “hard” confirmation of this.

It is as if the dBV and -10dBV into 10k quietly appeared, and it was expected that all concerned knew what they meant.

I first came across these in 1995. I needed some audio isolating transformers (to prevent hum loops in what was then a complex mix of audio and video equipment) and found that Sescom was a supplier of such. In fact, at the time, Sescom offered a variety of audio interconnecting units, not just transformers, intended to facilitate matching between diverse units, both professional and domestic. In its literature, the reference professional line level was given as balanced +4dBm into 600R, and the domestic reference was given as unbalanced -10dBV into 10k.

The first piece of equipment I encountered that used dBV in its specifications was the Panasonic NV-W1E standards-converting VCR, which I acquired 1995.

The audio line input was stated as -10 dBV, >50k.

The audio line output was stated as -8 dBV from <1k.

In practice the line output level of the NV-W1E was a good match for the 300 mV sensitivity inputs of the Quad 66, which is empirical evidence that the -8 dBV number represented typical programme, not full-scale. -8 dBV is 400 mV, which as a full-scale number would have better suited the 100 mV Quad 66 input.

Anyway, we may deduce that the dBV unit and the -10 dBV, 10k standard (whether de jure or de facto) was in place by 1995, but perhaps for not too long before that, otherwise it might have appeared in earlier specifications.

For example, a Sony TC-D5M cassette deck that I bought in 1993 had specifications following the established Sony practice, using both millivolts/volts and dB for the input and output levels, e.g. the line output was stated as 0.435 V (- 5dB). One may work out that the applicable 0 dB reference point was 0.775 V, and it was explicitly stated as such in the service manual, but strangely not in the operating instructions. But there was no mention of dBV.


Cheers,

Edit: Title should be "dBV, -10 dBV into 10k, etc."
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Old 19th Apr 2018, 8:14 am   #2
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Default Re: dBV, -10dV into 10k, etc.

Levels, impedance - it`s a whole can of worms.

I always think in terms of dBu as voltage wise it's the same as dBm (assuming 600 ohms) which is how most things used to be specified.
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Old 19th Apr 2018, 8:52 am   #3
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Default Re: dBV, -10dV into 10k, etc.

Quote:
Originally Posted by Synchrodyne View Post
When did dBV (where 0 dBV = 1 volt) come into use as a unit for measuring signal levels in domestic audio system interconnections? ...
I'm not sure about domestic usage but my HP3561A dynamic signal analyser (a pro audio instrument I think) dates from the very early 1980's and displays signals in dBV. Personally I wouldn't want to use any other unit. The dBV is, except for an unambiguous numerical conversion, essentially the volt, and the volt is well-defined. Any unit where I have to assume another number (typically an impedance) will inevitably lead to tears sooner or later when I find myself working with someone who's assumed a different value for that number. In my early days as a scientist I remember a trainer warning us against ever relying on assumptions with the phrase "ASSUME makes an ASS of U and ME".

To be fair, I suspect your main gripe is not with the unit itself but with the mess of different values for 'line level'. I often find myself working with customers who don't understand why, having coupled a 'line output' from a source (say a CD player) via a 'line-level' preamplifier (which turns out to have 12dB of gain at full volume) into a 'line input' on their 100W/ch power amp, they can't turn the volume control up more than 10-20 degrees before they're deafened.

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Old 19th Apr 2018, 1:39 pm   #4
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Default Re: dBV, -10dV into 10k, etc.

The dBu is my bread and butter. And it comes with a perfectly reasonable back-story, so I'm happy with that! My HP8903A gives results in dBV, so I have to mentally adjust by 2.21dB whenever I read from it. If I ever get the chance to buy an 8903B, I'll jump at it

As for "prosumer" standards, I'd love to know where they all came from. What comes out the back of a bit of gear on a phono socket really is anyone's guess. CD players were sort-of standardised at 0dBFS = 2V RMS, until manufacturers starting edging up the levels. Before that, levels were lower, and in the infancy of CD some hi-fi amps incorporated a "pad" on the CD inputs to avoid a jump in level when changing source. A lot of hifi tuners and cassette decks were around 400mV RMS at peak modulation, but again there was a lot of variation.

Of course, the programme material has changed wildly. Originally, 2V RMS from a CD player isn't unreasonable compared to other sources when you played music with a natural dynamic range (of around 20dB), even if the peaks are hitting full-scale. Fast-forward to the Loudness Wars of today and that becomes a problem. The same is true for FM radio, thanks to the Optimod. So it's really best to keep in the test-tone domain for audio level standards, IMHO.

In practice, does it really matter?

People adjust the volume to taste, and a few dB of difference between sources, CDs or radio stations is hardly an issue. I've always assumed that the -10dB standards are extremely nominal at best - in the professional environment these signals will always be adjusted as required, so providing we're no-where near clipping or the noise floor, then that's good
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Old 19th Apr 2018, 3:27 pm   #5
barrymagrec
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Default Re: dBV, -10dV into 10k, etc.

In my early days in audio with tape and film machines we always referred to dBm although we actually meant dBu , a term which was not in much use at the time. Even then (1969) though most professional kit was nominally 600 ohms, in practice this meant a much lower output impedance and normally a bridging input of 10k or so.

0 dBm (dBu) was 0.775 volts and peak level was +8. I don`t think these levels have changed in the intervening years but I have been away from the film and broadcasting business for more than 30 years now.

Most AC Millivoltmeters used for audio measurements, such as my Advance VM77 had a dB scale based on 0dB being 0.775 volts.

I don`t think there has ever been much in the way of standards in terms of levels for domestic equipment.
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Old 19th Apr 2018, 3:57 pm   #6
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Default Re: dBV, -10dV into 10k, etc.

I had always understood it as the input and output levels on home audio equipment were designed around the voltage levels available from a ceramic or crystal pick-up cartridge, so as not to need extra gain or attenuation switched in when using a tape deck or radio tuner; whereas professional equipment was designed around a standard level of 1mW into 600Ω = c. 0.78V.
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Old 19th Apr 2018, 5:28 pm   #7
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Default Re: dBV, -10dV into 10k, etc.

Definitions of audio levels are standardized by the Audio Engineering Society as follows:

0 dBu Preferred informal abbreviation for the official dB (0.775 V); a voltage reference point equal to 0.775 Vrms. [This reference originally was labeled dBv (lower-case) but was too often confused with dBV (upper-case), so it was changed to dBu (for unterminated).]

+4 dBu Standard pro audio voltage reference level equal to 1.23 Vrms.

0 dBV Preferred informal abbreviation for the official dB (1.0 V); a voltage reference point equal to 1.0 Vrms.

-10 dBV Standard voltage reference level for consumer and some pro audio use (e.g. TASCAM), equal to 0.316 Vrms. (Tip: RCA connectors are a good indicator of units operating at -10 dBV levels.)

0 dBm Preferred informal abbreviation of the official dB (mW); a power reference point equal to 1 milliwatt. To convert into an equivalent voltage level, the impedance must be specified. For example, 0 dBm into 600 ohms gives an equivalent voltage level of 0.775 V, or 0 dBu (see above); however, 0 dBm into 50 ohms, for instance, yields an equivalent voltage of 0.224 V — something quite different. Since modern audio engineering is concerned with voltage levels, as opposed to power levels of yore, the convention of using a reference level of 0 dBm is obsolete. The reference levels of +4 dBu, or -10 dBV are the preferred units.

0 dBr An arbitrary reference level (r = re; or reference) that must be specified. For example, a signal-to-noise graph may be calibrated in dBr, where 0 dBr is specified to be equal to 1.23 Vrms (+4 dBu); commonly stated as "dB re +4," that is, "0 dBr is defined to be equal to +4 dBu."

0 dBFS A digital audio reference level equal to "Full Scale." Used in specifying A/D and D/A audio data converters. Full scale refers to the maximum peak voltage level possible before "digital clipping," or digital overload (see overs) of the data converter. The Full Scale value is fixed by the internal data converter design, and varies from model to model.

As the dBm definition makes clear, the use of dBm is obsolete (thank goodness because it's very confusing to think of power levels rather than the usual voltage levels). With hindsight I joined the industry just as the relevance of 600 ohm standard impedances was dying out. The characteristic impedance of a pair of open telephone lines spaced 9 inches apart never did have much to do with studio audio! I believe the 600 ohm reference was largely to do with the understandable adoption of telephony engineering practice in the nascent broadcast technology of the 1920s.

As Julie implies, consumer levels seem to have developed simply as a practical signal voltage high enough to give a decent signal to noise ratio with typical valve preamps. I understand that the original choice of a higher level for CD players was intended to make the most of the available 90dB or so dynamic range, leaving enough headroom for occasional peaks in the programme content. However, it didn't quite work out that way, with CD producers pushing compressed average digital signal levels up towards the maximum 0dBFS. As a result, CD players tend to be too loud compared with other sources.

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Old 19th Apr 2018, 8:50 pm   #8
mhennessy
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Default Re: dBV, -10dV into 10k, etc.

The question of what the "u" in dBu actually stands for seems to be a matter of debate. As well as above, I've heard "undefined" - in that the impedance is no longer defined given that it's a voltage unit. I think that is as plausible as "unterminated", perhaps better in some respects...

As the relationship between dBu and dBFS is essentially an arbitrary choice made by the designer, there are standards available for him/her to choose from. The most common one in my experience is 0dBu = -18dBFS. This is a good compromise for 16 bit systems, but less so for 24 bit, where it doesn't use the extra bits to increase headroom and has a theoretical noise floor that can't be reached in practice. A good analogue sound desk will have perhaps 10dB more headroom than a digital system working to this standard.

It's worth saying that the definition of 0dBFS is based on a sine wave, so positive values of dBFS are possible (a square wave should reach +3dBFS in theory). That's ignoring inter-sample values generated in the reconstruction filter.

For levels in broadcast, 0dBu for lineup and +8dBu (PPM 6) for programme peaks still hangs around - especially in radio - but with the advent of loudness normalisation (R128) for television, it's all change!

Realising that the peak-level normalised world - itself a legacy of analogue equipment - is what led to the loudness wars in their various forms, television programmes now must be loudness level normalised. This is a huge paradigm shift that we're still coming to terms with, but if anyone has noticed that the adverts are no longer as loud as they used to be, that's why.

I have a series of audio tracks that convincingly demonstrate that once you have equalised playback level based on loudness (essentially the weighted average level), modern hyper-compressed recordings sound dull and flat, while the recordings with a natural dynamic range - which previously sounded too quiet - now have the advantage. So producers are slowing realising that now, to make your mix stand out, you have to make it more dynamic, not less.

Now the loudness level of a programme has to be -23dBFS. I've lied about the units - it's actually -23LUFS, but 1LU is equal to 1dB, and we already know dBs, right

As for peak levels, you are now allowed to get to -1dBFS. Again, I've lied about the units - it's actually -1dBTP (True Peak). To measure the true peak level, it's not enough to simply look at the sample values because you can get inter-sample levels that are higher. So you need to use an over-sampling process to do that accurately.

All this means that we can use more of the available dynamic range. Prior to this, signal levels were not allowed to exceed PPM6 (+8dBu, or -10dBFS). That should be qualified though: as a PPM has some integration time, it should be described as a QPPM (quasi peak programme meter). PPMs are known for under-reading on impulsive material - especially harpsichords and castanets, for example. But that came about because it was observed that brief bursts of distortion are not audible, so the 10ms integration time of "BBC" type II PPM permits a slight rise in average levels without obvious artefacts. But that was decided in the late 1930s, when there was much less HF content in programme material - today we have to be more careful... Either way, much as the PPM isn't perfect, lots of people are very good at interpreting it.

This stuff is surprisingly complicated. Of all the various issues encountered in broadcast audio, level ought to be simple, but is one of the hardest to explain and understand. I still find that surprising.

Away from conventional broadcast, big players like YouTube and iTunes have their own ways to normalise according to loudness levels. This gives me hope for the future of audio

Sorry Steve - none of this helps with the domestic question. As I say, nominal at best. I dread to think how many spec sheets I must have looked at over the years, and the output voltage specifications vary enormously - much as they (excluding CD players and other "2V" digital devices) might be in the 316mV ballpark, they fall in a 200-500mV range. I honestly wouldn't be surprised if someone just said "call it -10dBV as that's a nice, round number right in the middle of the apparent range" - and then others thought "that's a good plan - let's do the same!".

Last edited by mhennessy; 19th Apr 2018 at 8:55 pm.
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Old 19th Apr 2018, 9:15 pm   #9
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Default Re: dBV, -10dV into 10k, etc.

Quote:
Originally Posted by barrymagrec View Post
Levels, impedance - it`s a whole can of worms.

I always think in terms of dBu as voltage wise it's the same as dBm (assuming 600 ohms) which is how most things used to be specified.
I'd be careful on that line of thought, as dBm refers to gain as refered to 1 mW. But saying that , 1mW varies depending on the impedance of the circuit. ( which could be 600 /150/75/ 300 ohms ,or perhaps some other value )
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Old 19th Apr 2018, 9:34 pm   #10
barrymagrec
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Default Re: dBV, -10dV into 10k, etc.

That`s why I said assuming 600 ohms. some telecoms kit used various other impedances and of course 75 and 50 ohms are widely used for RF and data purposes but for audio use most people think 600 ohms - even though it mostly isn`t any more.

Better class Telecoms test equipment such as Siemens and Wandel and Goltermann switch meter sensitivity with load impedance so you always read the correct level in dBm whatever impedance your circuit is.
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Old 19th Apr 2018, 9:34 pm   #11
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Default Re: dBV, -10dV into 10k, etc.

Quote:
as dBm refers to gain as refered to 1 mW
I think you mean dBm refers to power relative to 1mW. barrymagrec was correct in saying that, assuming 600 ohms, dBu and dBm give the same voltage level.

NB things get even more exciting if we attempt to harmonise / work between all the various methods of expressing (audio) signal-to-noise .......
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Old 19th Apr 2018, 9:38 pm   #12
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Default Re: dBV, -10dV into 10k, etc.

dBm is a power level, so is independent of impedance. dB is gain, so is independent of power level.
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Old 20th Apr 2018, 12:57 am   #13
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Default Re: dBV, -10dV into 10k, etc.

Thanks much for all of the replies. It is good to have confirmation that -10 dBV is part of an AES standard. Is there any information as to when it was brought into that standard?

Quote:
Originally Posted by mhennessy View Post
As I say, nominal at best. I dread to think how many spec sheets I must have looked at over the years, and the output voltage specifications vary enormously - much as they (excluding CD players and other "2V" digital devices) might be in the 316mV ballpark, they fall in a 200-500mV range. I honestly wouldn't be surprised if someone just said "call it -10dBV as that's a nice, round number right in the middle of the apparent range" - and then others thought "that's a good plan - let's do the same!".
Yes, it does look as if that kind of reasoning was the basis for the -10 dBV level. I have looked at domestic equipment “line” output levels over the years, and I’ll revert with more on that. But it does seem that -10 dBV was a good approximation to typical analogue equipment levels of the 1980s and onwards on average programme. And of course, 316 mV is the geometric mean of the 200 to 500 mV range.

The associated 10k impedance looks like a CD-era number. Whereas CD players from the start usually had outputs that could drive such an impedance, 10k would have been far too low foir much earlier equipment.

The 1995 Secom data that I mentioned previously correlated -10 dBV with +4 dBm when it came from conversion between domestic and professional equipment:

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It’s interesting that although Sescom used dBV for domestic/semi-professional equipment, it was still using dBm, rather than dBu for professional equipment.

If we take the professional FS number as being +18 dBu, that is 14 dB above the +4 dBu reference level, then correlating +4 dBm professional level with the -10 dBV consumer level means that consumer FS is +4 dBV, or 1.6 V. I suspect that that wasn’t too far from what was actually experienced with early CDs.


Cheers,
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Old 20th Apr 2018, 6:40 am   #14
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Default Re: dBV, -10dV into 10k, etc.

Quote:
Originally Posted by G8HQP Dave View Post
dBm is a power level, so is independent of impedance. dB is gain, so is independent of power level.
Exactly!
Well said.
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Old 20th Apr 2018, 9:52 am   #15
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Default Re: dBV, -10dV into 10k, etc.

As an aside, I recall that the standard line programme level in Australia was +8 dBm. Not sure why it was so high. It used to require an output stage capable of producing some significant power, bearing in mind that it had to have a headroom of another 20 dB!
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Old 20th Apr 2018, 10:25 am   #16
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Default Re: dBV, -10dV into 10k, etc.

+8 dBm is standard peak level for 600 ohm systems.

6 on BBC PPM meter, + 4VU, which is off the scale, obviously.
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Old 20th Apr 2018, 11:59 am   #17
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Default Re: dBV, -10dV into 10k, etc.

But I was referring to the line up level. The (dreaded) VU meters were set to zero at +8. PPM's sadly, were hardly heard of.
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Old 20th Apr 2018, 12:29 pm   #18
barrymagrec
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Default Re: dBV, -10dV into 10k, etc.

Interesting, It sounds as if they use 0VU as an absolute peak level, unless the telecoms people permit a higher transmission level than +8dBm which I seem to remember was an absolute limit as far as the GPO / BT were concerned.
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Old 20th Apr 2018, 12:51 pm   #19
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Default Re: dBV, -10dV into 10k, etc.

Not sure about the level on the APO lines - I only worked on the transmissions withing studios. I do recall a BBC engineer being surprised at the level.
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Old 20th Apr 2018, 4:28 pm   #20
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Default Re: dBV, -10dV into 10k, etc.

Standard audio line level within Australian Aviation Systems (ATC) was 0 dBm in 600 ohms with a 1000Hz test tone.
Speech at this level indicates approx -8 dBm on a weighted level meter.
We lined up test tones into Telstra (P.O.) external lines 10 dB lower, ie at -10dBm test tone, and approx -18dBm indicated speech, to avoid distortion and to obtain a better frequency response.

Viva dBm!
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