Calibrate 0dB to 0dB

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JazzBo
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How do i calibrate 0dB everywhere in Ardour 2.6.1 with an sblive 5.1 card under Debian Lenny.

Capture line in shows about -18dB. This is regardless if it is sent -30dB or +4dB from an extern mixer. -30dBu output on the extern mixer shows peaks about -20dB on the meters in Ardour. +4dBu on the extern mixer shows -18dB in Ardour.

The mixers headroom is +10dB. A still clean signal +9dB on the extern mixers vu-meter and line out boosted +4dB (resulting on a crunchy sound on the sound card) shows in Ardour with peaks about -17dB.

Capture a cd show peaks about -5dBu in ardour. It might depend on the cd.

The reference voltage for the decibel unloaded (0 dBu) is the voltage required to produce 1 mW of power across a 600 Ω load (approximately 0.7746 Vrms)

It would be nice to know if it is possible to calibrate the levels. Perhaps it is an ALSA issue?

If someone is interested...
A sine wave at -10dBu is about 0.1 volt rms
A sine wave at +4dBu is about 1.228 volt rms
A sine wave at +6dBu is about 1.55 volt rms

-10dBu is consumer audio standard
+4dBu is American profesional audio standard
+6dBu is some German standard

regards
JazzBo

paul
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Once you are on the digital side of the analog/digital divide, discussions of dBu make no sense at all - only dBFS is really meaningful. This is because the mapping between absolute sample values (or dBFS if you want a volume-based unit) and dBu levels seen before A/D and after D/A are dependent on the converters, and is not adjustable from within the digital realm. I suspect that you know this.

Consequently, your question is really about doing calibration of individual cards and necessarily involves using both an analog measuring tool and a digital measuring tool to compare levels before and after conversion. It cannot be done with ALSA alone, just as it could not be done with a voltmeter alone.

Finally, I am not entirely sure that I trust the numbers you are getting from ALSA regarding mixer levels. Which ALSA tool are you using to get them?

JazzBo
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This is how i did

A mixer plugged in to line in on the sound card. Output levels from the mixer is -30dBu or +4dBu.

A keyboard plugged line in to the mixer and a pedal A2 (hit an A in the second octave, pres pedal to get it sounding into eternalty) to get something sounding.

This pedal A (a fairly sinus-like organ tone fairly close to 1kHz) is adjusted on the mixers vu-meter to 0dB. This signal is sent to a PA to see what's in the cable. This signal shows up at the PA as a 0dB signal. Everything seem OK. Mixer healthy. Signals sane. Sounding good. Don't know if my neighbours agree though.

Mixer out 0dB ==> PA in 0dB ==> Speaker AAA...

I have not used an oscilloscope to really see that the line out signal is really 0dB. I just accept that the mixer is sending 0dB as 0dB. And the PA accept this signal as 0dB.

The same cable goes to line in on the sound card.

Jackd.

Ardour.

Try to record something.

The signal ends up on Ardours vu-meter as -22dB. (21.7).

Mixer out 0dB ==> Ardour in -22dB.

JazzBo
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Well...

Perhaps Ardour should be regarded more as a shiny toy in an office computer than a tool for digital recording.

I don't know the signal/noise ratio on my sound card, but let's say it is 96dB. And let's say, what Ardours meters are showing is fairly accurate, then the recorded s/n would be about 74dB, or about the same as an old cassette recorder.

Does the vu-meters in Ardour show something that makes any sense at all?

The signal sendt to the sound card should have a SN ratio 115dB. Distortion is 0.005% at +4dBu, 1kHz according to the mixers data sheet.

paul
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Ardour, like just about all DAWs, doesn't have any VU-meters. It has digital peak meters that show all values in dBFS. There is no way to know the mapping between dBFS and dBU unless you have a very carefully calibrated audio interface. You cannot take measurements of the level before the audio interface - that device contains a gain stage, and on most audio interfaces, not a particularly accurately controlled one either. You could have a very precisely measured dBU level going into the soundcard, and a totally different level entering the device's A/D converters. You also don't know how the A/D converters are calibrated - this is also controllable on a few audio interfaces, but generally you just have to take what you get. Furthermore, many cheaper A/D converters are non-linear in this respect. Add all of this up, and you get what I said to start with - you have no way to know what a given dBU level will map to in dBFS on a given system. Put a different way: dbU is an analog level, dbFS is digital, and because the conversion between them is subject to several different parameters, you can't assign a fixed mapping.

Do you intend to keep on being insulting?

Felix
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I'm sorry, JazzBo, your ideal for Ardour is kinda sad.

I think it is a very good alternative especially for field recording operations where you need a small (often underpowered sometimes) notebook with an efficient OS (plenty of distros that have install sizes no bigger than the CD they come in) and program for optimum battery life (batt life of Asus EEE linux vs windows version for example).

JazzBo
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Do you intend to keep on being insulting?

Answer No!

It wasn't my intention to be insulting.

You don't trust the levels i get on the meters. That is OK with me. I guess i don't understand how to interpret the levels Ardour shows on the meters.

What i say next is not meant to be insulting either. I tried to do record the same thing with the same card under xp (dualboot) and kristal, a free recording prog. And with kristal it was possible get the peak meters follow the meters on the extern mixer fairly well. All the way to the final mix.

The same card under Linux and it is impossible to record with good sound quality and the peak meters showing more than -30 peakmeter units. And the final mix doesn't sound very loud.

There is in Ardour peakmeter units from well below -50, a 0 level and a level a bit over +4.

This Zero level seems to matter on my cheap card under DOS.

It is only line in that comes in as -30. And if you compensate the -30 and lower levels with higher volume out in the monitors it can get shockingly loud if a softsynth, or some imported wav file comes thundering in at 0 or +4 peaklevel units.

I don't care if i am wrong. There must be a way to get a zero db level that is zero db under Linux when it is possible under DOS.

And i am sorry to say (not insultingly) that i ran into a bundle of reproduceable crashes just by plain recording attempts.

qharley
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Ok, lets start from the beginning.

1) The Creative Audigy is not a card to use for any serious recording work.
2) The levels in ardour is only a representation of how it was received from Jack. Even if it peaks at -18 dB it is still ok, as ardour has 32bit float internally, and every 6db only represents a single bit of resolution loss. You are recording at 24 bit, so you can loose 4 bits without noticing...
3) the markings in the level meters are only for the slider positions, and not for the level. The peak level is displayed in the little block above the level meter.
4) If you want proper VU, load a VU and connect via Jack

0dB is meaningless in digital recording, because it indicates clipping, which you definitely don't want.

Record with peak levels at -18 to -12 db, and normalise if you want when done.
I don't normalise. After mixing, you will find that in most cases you have to tone down the faders in any case to prevent clipping on the master bus anyway.

qharley
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You did check the Alsa volume control? Sounds like the line-in might be turned down...

That is why I use the RME Hammerfall, with an ADAT converter interface for proper recording. I hate fiddly software controls hidden all over the show...

JazzBo
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Finally someone that understood the question.

I know Creative Audigy isn't for serious audio.

I use EMU 1820 in DOSland, emu1212 or if it is 0404. but it isn't fully supported in Linux yet.

Line in on the Audigy with peak levels over -31 got recorded with some fuzzbox like effect.

Then i fiddled with the Alsa mixer and suspect that it is a flaw there. 58 stereo sliders and sometimes more than one slider can alter the level of a given signal. Now i can get something recorded clean with peaklevels around -11.

That was the original question. how to get Ardour to agree on 0dB. Just as a reference.

thanks
JazzBo

paul
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As I explain very very frequently, Ardour does not interact with your audio h/w. This task is left to JACK, and even JACK generally avoids trying to interact with your hardware mixer. These mixers are highly device specific - its only by abstracting them down to the simplest common denominator that you can make a general interface to control them (which is what Windows does, and is what Kristal is using). Because many of our users use devices where such a simple abstraction makes no real sense (e.g. anything based on the ice1712/1724 chipsets like M-Audio or Terratec PCI cards, or the RME HDSP series), we choose to not try to represent this within JACK. We assume that you've configured your hardware with tools appropriate for the hardware, and that you're ready to go.

Andnow lets be very clear: using the hardware mixer is NOT getting Ardour to "agree" on a 0dB reference. What it means is that you controlling the analog gain stage of the your audio interface so that its A/D converters generate 0 dBFS when you send a 0dBU signal. This has nothing to do with Ardour, JACK, your CPU, operating system or anything else. Its getting the analog input into the converters within the range understood by the converters.