Record level calibration

Archived from groups: rec.audio.pro (More info?)

I've adjusted the playback electronics on my MX5050 so that when
playing back a 1k tone off the test tape, I get 1.228 v at the output
and show 0 on the VU meter. When I got to adjusting the record level,
the Otari manual had the following to say:


3-6-(4) RECORD LEVEL ADJUSTMENT

This adjustment should be made at 15 ips for high speed models and 7
1/2 ips for low speed models.

12. Set audio oscillator freqeuncy to 1kHz and output level -15dBm.
13. Set INPUT LINE control to full clockwise position. Set SRL switch
to SRL. Set MONITOR switch to TAPE.
14. Start record mode. Adjust REC LEVEL potentiometer for 0 VU on VU
meter.

3-6-(5) RECORD LEVEL CALIBRATION

This calibration should be made after the record level adjustment
continuously.

15. Set MONITOR switch to SOURCE position.
16. Calibrate Rec. CAL. potentiometer so that VU meter inicates 0 VU.



What's confusing me (aside from the "adjustment continuosly" ) is the
instruction to set the oscillator level for -15dBm. Given the rest of
the instruction, shouldn't it be set to 1.228 v (+4dBu?) so that 1.228
v in gives you 1.228 v out off the play head? Isn't that 19dB below
what I'd want to use? What am I missing?

Archived from groups: rec.audio.pro (More info?)

mrivers@d-and-d.com (Mike Rivers) wrote in message news:<znr1100795300k@trad>...
> In article <7cc3cba6.0411180633.3da55d04@posting.google.com> tacoma57@hotmail.com writes:
>
> > 3-6-(4) RECORD LEVEL ADJUSTMENT
>
> > 12. Set audio oscillator freqeuncy to 1kHz and output level -15dBm.
> > 13. Set INPUT LINE control to full clockwise position. Set SRL switch
> > to SRL. Set MONITOR switch to TAPE.
> > 14. Start record mode. Adjust REC LEVEL potentiometer for 0 VU on VU
> > meter.
>
> I don't have a manual or a recorder here to try to interpret this for
> you but I can see why it's confusing. When the SRL switch is in the
> SRL position, it should disable the line input level control and set
> it so that +4 dBu (or whatever you choose for your standard record
> level) can be calibrated for a playback level of 0 VU (which has
> previously been calibrated to +4 dBu (1.23 V).

Thanks Mike.
The SRL disables the Playback Level knobs on the front of the deck -
no effect on the Input Level knobs. The big Input Level knobs on the
front of the deck (the ones they say to turn full clockwise) do
interact with the internal "Record Level" trim pot. If I put 1.228
volts in with the Input Level fully clockwise, I can't trim the Record
Level back far enough to get 1.228 out (in fact the VU meter is
pegged). With the Record Level trim backed all the way down, I need to
back the Input Level knobs down about 1/3 of the way to get 1.228 on
output off the play head. It seems like I shouldn't have to back the
Record Level all the way down, so I suspect somethings wrong with this
approach.

> Perhaps with the SRL switch off and the input level control set fully
> clockwise, there's enough gain so that "-15 dBm" (which isn't a
> voltage level but it's a common mistake) will give the normal
> recording level.

How would I set the oscillator for -15dBm?

Archived from groups: rec.audio.pro (More info?)

mrivers@d-and-d.com (Mike Rivers) wrote in message news:<znr1100903045k@trad>...
> In article <7cc3cba6.0411190939.658d13b1@posting.google.com> tacoma57@hotmail.com writes:
>
> > The SRL disables the Playback Level knobs on the front of the deck -
> > no effect on the Input Level knobs.
>
> That's what I thought (my MX55 is the same way) which is why I was
> confused when it appeared in the instructions for setting the record
> calibration. I guess I just expected that you'd know to do that.
>
> > The big Input Level knobs on the
> > front of the deck (the ones they say to turn full clockwise) do
> > interact with the internal "Record Level" trim pot. If I put 1.228
> > volts in with the Input Level fully clockwise, I can't trim the Record
> > Level back far enough to get 1.228 out (in fact the VU meter is
> > pegged). With the Record Level trim backed all the way down, I need to
> > back the Input Level knobs down about 1/3 of the way to get 1.228 on
> > output off the play head. It seems like I shouldn't have to back the
> > Record Level all the way down, so I suspect somethings wrong with this
> > approach.
>
> This is why they tell you to apply a signal at less than +4 dBu when
> the level control is set fully clockwise.
>
> > How would I set the oscillator for -15dBm?
>
> By measuring the output with a voltmenter. And since the recorder has
> a relatively high input impedance (certainly not 600 ohms) you'd want
> to set the oscillator to -15 dBu, not -15 dBm. dBu is voltage measured
> into an open circuit, where dBm is actually a power measurement (dB
> relative to 1 milliwatt) and with audio equipment, generally is
> measured with a voltmeter with the generator operating into a 600 ohm
> load. Ohm's Law tells us that 0.775 volts across 600 ohms is 1 mW.
>
> But to answer your real question, set the output of the oscillator to
> 0.14 volts and you'll be close enough.

Got it, Thanks. I had to pad down the output of my oscillator, then
all seemd to go alright. Out of curiousity, why would the level for
the oscillator be given in dBm if the recorder's input is not 600
ohms?

Archived from groups: rec.audio.pro (More info?)

In article <znr1101251951k@trad>, Mike Rivers <mrivers@d-and-d.com> wrote:
>In article <7cc3cba6.0411231406.1d39ee9f@posting.google.com> tacoma57@hotmail.com writes:
>
>> Out of curiousity, why would the level for
>> the oscillator be given in dBm if the recorder's input is not 600
>> ohms?
>
>Ignorance. You can't even blame this on bad translation from the
>Japanese, because dBm is the same in both languages.

Everybody does it, though. It's meaningless, but common on data sheets.
The transition from power-transfer to voltage-transfer lines and measurements
has been going on for a long time and folks STILL aren't quite ready.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."