Design Article
2 Vrms - A funny old standard
By Dafydd Roche, Texas Instruments
9/4/2008 2:49 PM EDT
Traditional interface standards were given at +4 dBu and -10 dBV (that's 1.23 Vrms and 0.316 Vrms for those without a calculator or the will to make the calculations). For many years, this was what differentiated consumer and professional equipment. Take any standalone CD player from the early 1990s and you'll quickly see the standard I'm talking about.
In the late 1980s and early 1990s people got a little pickier about what they connected to their television. The days of just hooking up the VHS machine via the coaxial aerial connection was slowly coming to an end. With the advent of higher quality audio sources (NICAM stereo, Laserdisc, etc.), audio started being transported across stereo RCA jacks - just like your Hi-Fi did.
SCART standard
Around the same time, SCART connectors in Europe started to become popular, too (Figure 1). SCART gave manufacturers a whole new playground of standards to play with. Not only were the connectors new, but the levels that people could play increased significantly.
Figure 1 - SCART Connector.
As with any interconnect, the higher the signal you push through, the higher the dynamic range you can get in the receiver as receivers typically have a fixed amount of noise. Along with the new connector came standards for voltage swing and for the expected impedance a receiver should impose on the driver. The SCART standard eventually grew to be 2 Vrms (5.6 Vpp) with an input impedance of 10 kΩ.
A ground-biased 5.6-Vpp signal is tougher to generate than one may at first think. Consider for a moment the products we connect in a home entertainment environment: DVD players, set-top boxes, gaming consoles, AV receivers, etc. The output of a typical 5-V DAC is around 4 Vpp, biased around 2.5 V. Lower operating voltage DACs will have even less signal swing biased around a lower voltage. Getting this signal into the outside world requires some level of gain and a level of buffering.
This nicely brings us to output topologies for taking DAC outputs to the real world. There are three main methods of doing this:
- bipolar-supply operational amplifier (op amp) with a split rail power supply;
- single-supply op amp with a DC bias of VDD/2, and a DC blocking capacitor on the output;
- single-supply op amp with a built-in charge pump to generate a negative supply rail, giving enough swing and a ground-biased output.
Each of these topologies has their own advantages and disadvantages.
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EricStromberg
9/5/2008 11:39 AM EDT
Actually, the traditional standards are +4 and -20. -10 didn't come along until the MIDI keyboards of the early 80's.
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The Artful Bodger
9/6/2008 2:29 AM EDT
Actually, first, the 2VRMS standard for 0dbfs came in with the very first commercial CD-Player (the Philips CD-100 see technical data @ http://www.marantzphilips.nl/index.asp?strPage=Info&strBrand=Philips&strType=Philips_cd100).
So looking at an old (or current) HiFi CD-player will always reveal 2VRMS @ 0dbfs excepting a tiny minority of cases.
As most traditional analog system allowed as much as 15db headroom above the notional 0db point; 2VRMS @ 0dbfs is actually close to around -10dbV with 15db headroom, when translated back into traditional analog systems numerology.
(For example LP, 0db is set as 5cm/s but maximum levels are standardised as 25cm/s or +15db; I cannot now readily remember the old modulation index of FM radio for "0db" but it was way below 100% too and open reel tape allowed at least 10db headroom in the 300Hz - 3Khz region, leading to many recording engineers running tape machines with the VU Meters [hence virtually useless meters] pegged most of the time at louder passages)
This then is the ultimate source of the 2Vrms standard, it was set as nearest full value at least 15db above 0db consumer standard (around 0.3V/-10dbV) of the time, so loudness levels would be comparable.
The loudness race starting in the late 80's and continuing up to today has shifted the average level on CD (and other digital recordings) progressively upwards from the -15dbfs baseline at the cost of ever more reduced crest factors and musical realism.
But that as they say is another story.
Basically a kind of "around 1-2Vrms @ maximum modulation" standard was already implicit in old analog media 0.3V @0dbvu due to headroom requirements and was formally codified with the arrival CD, removing any headroom consideration (which may vary depending on medium) from the equation.
Non of this makes ay of the challenges to fulfill the necessary requirements any less interesting though.
Even more interesting are the now common requirements in pro-audio equipment of presenting levels of +20dbu (7.75V RMS!) for 0dbfs. This is an area where line drivers with build in stepup from 5V DC (USB bus power) to +/- 15V internally could help majorly reducing BOM items and costs.
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10/11/2008 6:19 AM EDT
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The Artful Bodger
10/13/2008 7:01 AM EDT
Hi,
> The main thing I would like feedback on, now that you're engaged is discussion regarding this, is what you both think of
> directpath technology - the amplifier and the charge pump in the same package etc.
From a BOM/COST/Complexity view it seems a great idea.
I am not sure how good it is to have on-chip an chargepump with a frequency quite close to the audio band and with frequencies that can couple nicely.
If all the problems (potential or real) inherent to having these things on chip can be fully solved, it would be rather neat.
And an option for 7.75V RMS full scale balanced "pseudo transformer" inputs and outputs that can run directly of a single 5V supply at low cost would amke a lot of studio gear manufacturers quite happy....
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searing
8/28/2009 12:46 PM EDT
Another disadvantage to Solution #2 is the potential for microphonic problems. All three capacitors (C1, C2 and C3) have a voltage bias, and therefore if Class 2 or 3 ceramic capacitors (Y5V are the worst I've run into) are used, microphonics (at some level) would be expected. The microphonic noise pickup may be low, but there are other issues with capacitors that affect the sound quality.
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Guru of Grounding
9/4/2009 7:18 PM EDT
I'm late to this thread, but I have 2 comments:
1. Mr. Roche says "Traditional interface standards were given at +4 dBu and -10 dBV. For many years, this was what differentiated consumer and professional equipment." In fact, the most important difference between consumer and pro interfaces is that the consumer interface is unbalanced (with its inherent common-impedance noise coupling problems) and the professional interface is balanced (which can, if properly implemented (another pet peeve of mine), reject noise due to driver-receiver ground voltage differences).
2. On the subject of "proper" balanced input stages, the example in Figure 50 of the TI data sheet for the PCM4222 is a prime example of a circuit that will produce truly awful CMRR when connected to real-world equipment because its common-mode input impedances are far, far too low! I've published several AES papers on the subject ... but they never seem to reach the folks that need to know. I'll happily send copies to anyone interested - e-mail me at whitlock@jensen-transformers.com.
Bill Whitlock, President & Chief Engineer
Jensen Transformers, Inc.
AES Life Fellow, IEEE Senior Member
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