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AC coupling: recorder line out to VU meter

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Anonymous
March 1, 2005 12:42:22 AM

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

I have a portable meter without a rec level indicator on it so I built
a little VU meter with an LM3916 chip and a bargraph LED display. I'm
not going for ultimate accuracy, just gross levels. My line out on the
recorder has a bit of DC offset with I plan on sending to ground
through a simple low pass filter. i.e. I'm AC coupling the output of
the line out with the input of the VU meter. What would reasonable
capacitor and resistor sizes be for this application. Since I've not
put an output to speakers on the meter (I have a separate headphone
jack for that), I don't care about hi-fi). Just want to choose
appropriate R and C values. Thanks...
Anonymous
March 1, 2005 12:49:02 AM

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

oops, meant to say "I have a portable recorder..." (not portable meter)
Anonymous
March 1, 2005 1:20:39 AM

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

sehochst@chartertn.net wrote:

> oops, meant to say "I have a portable recorder..." (not portable meter)
>


And I think you meant "high-pass". Any value of C will block the DC
from the input. Without knowing more about your input circuit and
desired frequency response the correct values are not guessable.

But hey, try 47 uF and 100 k. Point the plus on the cap toward the more
positive voltage (you didn't say which way the output was offset).
Related resources
Anonymous
March 1, 2005 7:54:54 AM

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

Ah yes, I meant high pass. Late night. My input circuit is coming
from an iRiver H340. The manual says that the outputs are either 16
ohm/20mW or 32 ohm/12mW. I'm not sure which, because it doesn't
specify. I don't know any more than that. My desirable frequency
response would be 20 - 20kHz. The offset is positive and it starts as
a low offset and it creeps up into the 80 mV range (I stopped measuring
after about 5 minutes). Would you do anything different on R and C (or
anything else for that matter) with this info?
Anonymous
March 1, 2005 8:49:17 AM

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

So, instead of a series RC to ground and outputting from the center of
the two, you would simply use 2 caps, outputting to the VU meter from
the two?

>From the LM3916, the input is simply stated as "high impedance".
Anonymous
March 1, 2005 10:02:10 AM

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

Ah, but according to National Semiconductor, it can be a VU meter, but
not the way I've it configured. See:
http://www.national.com/ds.cgi/LM/LM3916.pdf Figure 7 shows the input
buffer that would apparently allow the chip to act as a VU meter. It
also has some input circuits that allow it to act like a PPM (DIN spec
45406) on page 12, Figure 6.

You might be able to divine some input impedance specs from this
document as well. I have the 3916 wired in the simple way shown on
page 7 of the PDF I linked. I will put one on each channel for
independent level monitoring. It will behave as an qualitative peak
and average level detector/meter wired in this way. When I input a 0dB
1 kHz sine wave from my iRiver, it registers about 3dB down which is
good enough for my application. In a few test recordings with voice,
it was able to give me enough information to prevent clipping. Also on
page 7, you'll see the simplified input circuit diagram (pin 5 = signal
in).
Anonymous
March 1, 2005 10:13:26 AM

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

Thanks... See my reply to Scott's post. I can easily light up all 10
LEDs that I've attached it to... [insert smile here] When I adjust the
rec level on my recorder, the meter seems to be doing it's job
adequately. If everything were set to spec, the sensitivity range
would be -20dB to +3 dB over a the reference voltage range (0 to +1.2V
as I've wired it). From this doc:
http://www.national.com/ds.cgi/LM/LM3916.pdf (page 7 shows the voltage
divider resistor values that bridge the comparators) the -20dB
threshold will be +85 mV (by using Ohm's law), +269mV for the second
LED, and so on.
Anonymous
March 1, 2005 10:38:14 AM

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

I will never call any implementation of the LM3916 a VU meter again.
I will never call any implementation of the LM3916 a VU meter again.
I will never call any implementation of the LM3916 a VU meter again.
I will never call any implementation of the LM3916 a VU meter again.
I will never call any implementation of the LM3916 a VU meter again.
I will never call any implementation of the LM3916 a VU meter again.

At least, I'll try not too...

Thanks for your help...
Anonymous
March 1, 2005 11:08:13 AM

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

sehochst@chartertn.net wrote:

> Ah yes, I meant high pass. Late night. My input circuit is coming
> from an iRiver H340. The manual says that the outputs are either 16
> ohm/20mW or 32 ohm/12mW. I'm not sure which, because it doesn't
> specify. I don't know any more than that. My desirable frequency
> response would be 20 - 20kHz. The offset is positive and it starts as
> a low offset and it creeps up into the 80 mV range (I stopped measuring
> after about 5 minutes). Would you do anything different on R and C (or
> anything else for that matter) with this info?


No, but by input circuit, I meant the input to your bargraph.
Particularly, its impedance. If it's anything else besides the chip
itself it would be knowable, but it's strangely missing from the chip's
spec sheet. Guessing it's somewhat more than 100k, that resistor value
more-or-less fixes it for you.
Anonymous
March 1, 2005 11:09:17 AM

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

In article <1109655742.544138.271620@l41g2000cwc.googlegroups.com>,
<sehochst@chartertn.net> wrote:
>I have a portable meter without a rec level indicator on it so I built
>a little VU meter with an LM3916 chip and a bargraph LED display. I'm
>not going for ultimate accuracy, just gross levels. My line out on the
>recorder has a bit of DC offset with I plan on sending to ground
>through a simple low pass filter. i.e. I'm AC coupling the output of
>the line out with the input of the VU meter. What would reasonable
>capacitor and resistor sizes be for this application. Since I've not
>put an output to speakers on the meter (I have a separate headphone
>jack for that), I don't care about hi-fi). Just want to choose
>appropriate R and C values. Thanks...

Well, you want to filter out only the DC, so you want the corner frequency
to be as low as possible.

That means you want as large a cap as possible going into as high a
resistance as possible. Skip the resistor and just use a coupling cap.
The value of the coupling cap depends on the input impedance of the chip
you're going into... if the input impedance is 6k ohms, a 1 uF cap
will be 3 dB down at 20 Hz. Capacitance and resistance will scale linearly
so you can work out anything from there.

I'd probably pick something like a 220 uF to be safe, but work the math
out and see for yourself.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
March 1, 2005 12:42:31 PM

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

sehochst@chartertn.net wrote:
> Thanks... See my reply to Scott's post. I can easily light up all 10
> LEDs that I've attached it to... [insert smile here] When I adjust
the
> rec level on my recorder, the meter seems to be doing it's job
> adequately. If everything were set to spec, the sensitivity range
> would be -20dB to +3 dB over a the reference voltage range (0 to
+1.2V
> as I've wired it). From this doc:
> http://www.national.com/ds.cgi/LM/LM3916.pdf (page 7 shows the
voltage
> divider resistor values that bridge the comparators) the -20dB
> threshold will be +85 mV (by using Ohm's law), +269mV for the second
> LED, and so on.

If your using the circuit shown on page 7, a 0.1uF DC blocking cap in
series with the input will do fine.

Mark
Anonymous
March 1, 2005 12:43:17 PM

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

<sehochst@chartertn.net> wrote:
>So, instead of a series RC to ground and outputting from the center of
>the two, you would simply use 2 caps, outputting to the VU meter from
>the two?

Right, one blocking cap in series on each side.

And this is NOT a VU meter. Don't call it one.

>>From the LM3916, the input is simply stated as "high impedance".

That's not very helpful. But that's one of the ways you can tell it's
not a real VU meter.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
March 1, 2005 12:51:50 PM

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

Scott Dorsey wrote:

<snip>

Skip the resistor and just use a coupling cap.

<snip>

I tried this with values of 10uF to 470 uF. I just get slow drift of
positive voltage (see original post - my line out drifts positive) at
the other end of the cap. This is the only way I get the drift to stop
at the input of the meter:

line out -------||-----/\/\/\-------meter input
|
|
GND

Where C is small (0.1 uF is what I'm using) and R is big (1M ohm).
This seems to allow good frequency response (I again wish I had an
o-scope) and LEDs from lighting when the iRiver is paused or during
quiet sections of music.
Anonymous
March 1, 2005 12:57:48 PM

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

In article <1109655742.544138.271620@l41g2000cwc.googlegroups.com> sehochst@chartertn.net writes:

> I built
> a little VU meter with an LM3916 chip and a bargraph LED display. I'm
> not going for ultimate accuracy, just gross levels. My line out on the
> recorder has a bit of DC offset with I plan on sending to ground
> through a simple low pass filter. i.e. I'm AC coupling the output of
> the line out with the input of the VU meter. What would reasonable
> capacitor and resistor sizes be for this application.

You don't send the DC to ground with a low pass filter, you block it
with a capacitor. The value of the capacitor is determined by the
input impedance of your meter - you want it to be large enough so that
the high pass filter formed by the input capacitor and input resistor
passes the lowest frequency that you want your meter to indicate.

Lacking any further information, I'd say that 20 uF should do it. That
will probably be an electrolytic. Unless the DC offset is substantial,
you'll probably want to orient the electrolytic with the + lead
connected to the meter input.

Do you have any idea what the input sensitivity of your meter is? Will
you shortly be asking us how to make it go higher than the second LED?



--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
Anonymous
March 1, 2005 1:00:16 PM

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

My picture didn't show up well when posted. The ground connection
should be between the R and C.
Anonymous
March 1, 2005 1:07:39 PM

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

<snip>

Skip the resistor and just use a coupling cap.


<snip>


I tried this with values of 10uF to 470 uF. I just get slow drift of
positive voltage (see original post - my line out drifts positive) at
the other end of the cap. This is the only way I get the drift to stop

at the input of the meter:


line out -------||-----/\/\/\-------meter input
|
|
GND


Where C is small (0.1 uF is what I'm using) and R is big (1M ohm).
This seems to allow good frequency response (I again wish I had an
o-scope) and LEDs from lighting when the iRiver is paused or during
quiet sections of music.
Anonymous
March 1, 2005 1:12:15 PM

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

Scott Dorsey wrote:
<snip>


Skip the resistor and just use a coupling cap.


<snip>


I tried this with values of 10uF to 470 uF. I just get slow drift of
positive voltage (see original post - my line out drifts positive) at
the other end of the cap. This is the only way I get the drift to stop



at the input of the meter:


line out -------||-----/\/\/\-------meter input
|
|
GND


Where C is small (0.1 uF is what I'm using) and R is big (1M ohm).
This seems to allow good frequency response (I again wish I had an
o-scope) and LEDs from lighting when the iRiver is paused or during
quiet sections of music.
Anonymous
March 1, 2005 1:16:55 PM

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

<sehochst@chartertn.net> wrote:
>Ah, but according to National Semiconductor, it can be a VU meter, but
>not the way I've it configured. See:
>http://www.national.com/ds.cgi/LM/LM3916.pdf Figure 7 shows the input
>buffer that would apparently allow the chip to act as a VU meter. It
>also has some input circuits that allow it to act like a PPM (DIN spec
>45406) on page 12, Figure 6.

No. The ballistics are NOT identical to a real VU meter. A real VU
meter has a particular input impedance as well, and it's not a linear one,
and it needs to be bridged across a 600 ohm line through a 3600 ohm
resistor. The specs for the VU meter are very, very tight and date back
many years to the days of iron-core meter movements.

>You might be able to divine some input impedance specs from this
>document as well. I have the 3916 wired in the simple way shown on
>page 7 of the PDF I linked. I will put one on each channel for
>independent level monitoring. It will behave as an qualitative peak
>and average level detector/meter wired in this way. When I input a 0dB
>1 kHz sine wave from my iRiver, it registers about 3dB down which is
>good enough for my application. In a few test recordings with voice,
>it was able to give me enough information to prevent clipping. Also on
>page 7, you'll see the simplified input circuit diagram (pin 5 = signal
>in).

Well, try a 20 uF cap. Put a 10 Hz signal and 1 KHz signal into it at
the same level. If they read the same, you're fine. If the 10 Hz signal
reads low, use a larger cap.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
March 1, 2005 1:17:44 PM

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

Scott Dorsey wrote:


<snip>

Skip the resistor and just use a coupling cap.


<snip>


I tried this with values of 10uF to 470 uF. I just get slow drift of
positive voltage (see original post - my line out drifts positive) at
the other end of the cap. This is the only way I get the drift to stop



at the input of the meter:


line out -------||-----/\/\/\-------gnd
|
|
to meter


Where C is small (0.1 uF is what I'm using) and R is big (1M ohm).
This seems to allow good frequency response (I again wish I had an
o-scope) and LEDs from lighting when the iRiver is paused or during
quiet sections of music.
Anonymous
March 1, 2005 1:45:45 PM

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

sehochst wrote ...
> My picture didn't show up well when posted.
> The ground connection should be between the R and C.

That would short ALL of your signal (AC and DC) to
ground and you would see nothing at the meter input.

I classic high-pass filter (to allow AC while blockingDC)
would be wired with a series capacitor (as you have) but
a parallel resistor (resistor to ground instead of a dead
short as you have shown). Your series resistor appears
to have no beneficial effect, and indeed is contributing to
your capacitance charging phenomenon.
Anonymous
March 1, 2005 2:36:51 PM

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

You may have seen the original post before I removed it because I had
drawn it incorrectly...

Here's what I meant to post (and later posted in another entry):

line out -------||-----/\/\/\-------gnd
|
|
to meter

Unless I'm have a severly dyslexic(sp?) day, the meter is between the
0.1uF cap and 1 Mohm R. The signal bounces along merrily on the meter
with no charging or whatever.
Anonymous
March 1, 2005 2:57:58 PM

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

Ah, this is what I meant to draw before I removed my post and replaced
it with the correct one. Your RC values have a -3dB frequency of
1.6Hz, so it should probably be fine. My values of 0.1uF and 1Mohm
yield the same -3dB point... Looks like we have a winning input with
reasonable RC values...
Anonymous
March 1, 2005 4:36:58 PM

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

In article <1109701064.203907.142660@z14g2000cwz.googlegroups.com>,
<sehochst@chartertn.net> wrote:
>
>I tried this with values of 10uF to 470 uF. I just get slow drift of
>positive voltage (see original post - my line out drifts positive) at
>the other end of the cap. This is the only way I get the drift to stop

Yes, this is the cap charging up. Use a smaller value cap. You probably
have a very high input impedance.

If you count how long it takes to rise a given measured amount, you can
figure the input impedance of the device this way too.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
March 1, 2005 10:09:21 PM

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

<sehochst@chartertn.net> wrote in message
news:1109690006.268154.58920@o13g2000cwo.googlegroups.com...
> Thanks... See my reply to Scott's post. I can easily light up all 10
> LEDs that I've attached it to... [insert smile here] When I adjust the
> rec level on my recorder, the meter seems to be doing it's job
> adequately. If everything were set to spec, the sensitivity range
> would be -20dB to +3 dB over a the reference voltage range (0 to +1.2V
> as I've wired it). From this doc:
> http://www.national.com/ds.cgi/LM/LM3916.pdf (page 7 shows the voltage
> divider resistor values that bridge the comparators) the -20dB
> threshold will be +85 mV (by using Ohm's law), +269mV for the second
> LED, and so on.

I suspect the input of the LM3916 wants to see a DC path to ground. Try
this:

1uF
o---------||---------------------->LM3916 input pin
|
|
100k resistor
|
|
Gnd

Make the cap mylar or polypropylene. Test; if 1kHz and 10Hz measure the
same, you're cool; if 10Hz is down, increase the value of the cap.

Peace,
Paul
Anonymous
March 1, 2005 10:10:46 PM

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

<sehochst@chartertn.net> wrote in message
news:1109700016.738431.151240@g14g2000cwa.googlegroups.com...
> My picture didn't show up well when posted. The ground connection
> should be between the R and C.

Well, that will certainly stop the drift. It shorts the signal to ground,
and the input of the 3916 too. No drift, no signal.

Peace,
Paul
Anonymous
March 1, 2005 10:43:48 PM

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

In article <1109689330.575932.137710@o13g2000cwo.googlegroups.com> sehochst@chartertn.net writes:

> Ah, but according to National Semiconductor, it can be a VU meter

Not really. A VU meter needs to have a certain scale (which is sort of
does since the progression of the LEDs is logarithmic, sort of) and it
has to have a certain ballistic response to transients, which it
doesn't. It also has a defined input impedance and is intened to
be connected across the source through a 3600 ohm resistor. But
that shouldn't stop you from using it as a signal level indicator, which
it does pretty well.


--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
!