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Topic: Adjusting the sensitivity of an audio timecode input (Read 281 times) previous topic - next topic

jackytwoshoes

I'm making a SMPTE LTC reader, which is working great, but I'd like the input circuit to have a little more sensitivity. Right now I have to use an amplifier to boost the signal.

Could someone advise how to change the input sensitivity of this circuit or perhaps integrate an op-amp to boost the input.

Thanks!



DVDdoug

Do you have any information about the SMPTE LTC reader?   Something about it's output signal?  Is this an industry standard we can look up?  Do you have a schematic of the reader?      We really need to know what's "wrong" with the signal...  Does it need to go more positive, more negative? both?

It's odd to see a capacitor in series with a digital signal.   Maybe taking the capacitor out will help???  Or, is this not a digital signal?

An op-amp is the easiest way to build an amplifier, and you could probably eliminate the transistor circuit.  An op-amp as a comparator or Schmitt trigger circuit might work too.  But, an op-amp usually requires a bi-polar power supply which can be an issue if your circuit doesn't already have one.    You can buy a Schmitt trigger chip, but these chips have fixed switching thresholds which might not be right for this applicatino.  And, I have no idea if that's even the solution since I don't fully understand the problem.

Or, maybe you can use an analog input on the Arduino?     You can compare the level to a threshold stored in software and pretty-much make it as "sensitive" as you like.   You'd have to get rid of the transistor (since it's output isn't linear/analog), but you'd probably need to keep the input transistor and "protection diode", and maybe add another diode and/or resistor depending on what kind of signal we have.

jackytwoshoes

It's a digital signal designed to be playable on any analogue audio device,  perhaps a bit old now but used loads for syncing cameras etc, or for editing video.

Explained here better than I could:

http://www.philrees.co.uk/articles/timecode.htm

I'm outputting from a 1/4 inch reel to reel machine that has a  0.775V (line) level output. The LTC is recorded at +1dB and isn't loud enough. I'm currently using a headphone amp to boost the signal.

Pelleplutt

If it works, go on with the headphoneamplifier, all suggestions you can get from us is a simple amplifier like a headphone amp.

Pelle

jaredcrogers

Hi jackytwoshoes,
Glad to hear yours is working!

I'm trying to use the same circuit for a timecode reader, but I think the code might be incomplete. 

Can you post the code you were using? 

Thanks!

Jared

Paulcet

I think halving the 10k on the transistor base would be a good start. 

Boardburner2

And does the diode on the base not clip the sicnal anyway before it gets amplified ?

Riva

I think you should be okay by just adding bias to the LTC input like in this...


An interesting (to me) project as the first microcontroller code I wrote (for the Microchip PIC) was to read and decode the LTC from Sony VTR machines. This was several years ago now and the code was written in assembler.
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