Hello everyone,

As my name suggest, I'm new to much of this and am learning about many things electronic so sorry if my questions seem not very smart. I've searched these forums and google and haven't found much on this (probably because the search terms pull in loads of unrelated things).

I'm trying to follow the directions to bias a reel to reel recording deck to a specific tape. The process is simple, but I need to feed the deck a signal at 7khz at -18DB.

I however, lack a signal generator.

"There's an app for that" Apple tells me, and indeed there is an app called Signal Generator which has a frequency and Gain (dB) adjustment.

Should I trust this thing?

The thing that really gets me is that the volume of the phone impacts the amplitude, but doesn't change the gain in the app. I'm guessing it should be used at full volume. (But who knows...)

(As an aside, I'm really struggling to understand the dB system in measuring gain. It seems relative to me, and something like the strength of the signal in mA would make more sense, but obviously I'm just not understanding since that's not how it's done.)

(If the app is no good, what's the most cost effective way of getting what I need. Could I build my own 7k Hz generator with a 555 chip or something? Again, how to get the proper gain?)

Thanks in advance for any help and guidance.

Luke

 

What you need to understand is that nothing measured in dB represents anything on an absolute scale. Everything measured in dB represents a magnitude relative to something else. Every time you see something measured in dB you have to find out what the reference is. If you do anything else you are just spinning your wheels.

Ronald Reagan was famous for the words "Trust, but verify". He was talking about nuclear weapons, but you get the point.

 

Okay, that actually makes perfect sense to me!

This deck has a VU meter (2 actually), so I need to apply an input that will register -18db on those meters.

Thanks for the answer, that made it click for me in the way it didn't before.

Basically the procedure is to "apply a 7k signal at -18dB" then adjust a trimmer capacitor to find a peak in the signal, then adjust (in a particular direction) until you get a dropoff of 3dB.

The dial in the iPhone app means 0dB is the maximum output at the given volume setting, then any -dB is relative to that. This makes sense. Let me know if I've misunderstood something though, but I think I've got it.

Thanks again,

Luke

 

Hello,

As @Papabravo said, dB's are relative.
You will need a reference for the scale.
Read this page for more info:
http://www.animations.physics.unsw.edu.au/jw/dB.htm
This is only one link from the EDUCYPEDIA page on Decibels:
http://educypedia.karadimov.info/electronics/decibels.htm

Bertus

 

I've used signal generators on my iPhone and I'm nearly certain that at least one of them was designed to produce a specified voltage at the headphone jack. I think it's purpose might have been for hearing testing, or maybe calibrating other equipment. Anyway I can look into that a little if you think it would help. But it sounds like you may have already found a protocol that will work for you.