signal generator

[mattias]

hey,

wouldn't something like this work for converting the direct flash output to a more recording friendly signal that won't interfere with and pollute the other channel? the battery is a one volt one, the resistor a one kiloohm one and the capacitor something i didn't bother figuring out. someone please correct me if those are the wrong numbers.

this was my thinking: when the contact is open, no current can flow through the circuit, so the output is zero. as soon as you the contact closes, the capacitor discharges through it, creating a voltage on the output. and when it opens again, the battery will charge the capacitor again, creating a negative voltage on the output, and we're back where we started. this should result in a clean signal meeting all the requirements of any line input, right?



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[mattias]

the signal should look like this:

[mattias]

maybe that should have been 60 ohms? damn, what are the specs for line and mic level outputs? i always forget. pedro?

/matt

[#Pedro]

Matthias,
line in and outputs work at 47 kOhms at 775 mV for 0 dB reading. They need and AC voltage (audio) as the frequency response is not garanteed down to 0 Hz. Only DC coupled measuring amps can process and amplyfy DC voltage. The next point is the recording. Depending on the recording technology, DC spikes as you are trying to create may be critical and not convinient thinking about s/n ratio, reliability of signal and so on.

Your circuit will react as following:
The recorder input (usually not line but mic in consumer devices) will get a power-on spike when connecting the circuit without camera running, due to the capacitor discharge. Afterwards it keeps galvanicly uncoupled and nothing will happen.
With camera running, your flash contact will SHORT CIRCUIT your battery every time it closes. Depending on the battery power and the flash contact, the contact may even burn and the battery may discharge quickly due to the direct short. This may produce regular DC-spikes, until the battery is down.

I cannot understand, why everybody is bothering with these pulse interfaces. Standards had been established e aprooved since pulse synchronisation exists and reliable circuits are cheap, small and easy to built. Why does everybody recuse feeding the audio input with a kind of signal that this input really expects, a real audio signal? It?s not much more dificult than bothering around with DC spikes, recorders are not designed for and would give you the reliability needed. The circuit may even fit into a DIN connector cap.
Go over 1000 Hz bursts and abandon the DC spikes, care for a secure 0 dB recording to get the best s/n ratio possible and you?ll have no troubles in resolving the audio and can concentrate on film and not on electronics.
As I have no webpage from where include pics into the posts, I am sending you the circuit in a email.
Pedro

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[mattias]

> I cannot understand, why everybody is bothering with these pulse interfaces.

because i can. i'll probably never build this thing, but how much do i learn about audio electronics by following your diagram? ;-) seriously, this is why i stick with super-8, even though i've really "moved on" to 16 and dv a long time ago. not because it's cheap, looks cool or is accessible. it's the "techno creative" thinking it forces me to use that keeps me hooked.

/matt

[Cranium]

"it's the "techno creative" thinking it forces me to use that keeps me hooked."

Bingo...
It is a great thing. It makes one think (maybe too much!) It drags the technical creativity out of you, and damn it's cool to find a great camera in a thrift store.
Maybe I'll change back to DV when I find an XL1-S with the Canon 3D lens at Goodwill.


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Mikel Z

[#Pedro]

"techno-creative" is very beautiful and exactly what I like, too. But technics/electronics are a materia following very strict "laws and orders". If you don?t obey theses basic things, you won?t be successful. Therefore, you can be very creative in realizing a huge amount of feeding circuits for pulse recording.
But the basic thing is the fixed interfacing to an audio recorder of whatever kind. So you MUST create an audio signal anyway, because it?s an audio recorder.
How you do this, depends on your creativity and the performance you wanna have.
You may use two transistors connected as an oscillator, any TTL oscillator circuits, the 555 timer circuit like I do and so on.
No limits, but the output must be sth like audio pulses resulting in a 0 dB recording.
I always try to learn, looking how profs are resolving the things, at least to confirm that I am on the right way.

Pedro

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[mattias]

well, i actually think my circuit will output a 0 db audio wave, at least after a small modification i've made that prevents short circuiting of the battery. only one period per pulse, and not at a very well calibrated frequency, but that's beside the point. or what would you suggest the lacking properties would be?

[Cranium]

"But technics/electronics are a materia following very strict "laws and orders". If you don?t obey theses basic things, you won?t be successful."

Awww.. But that's where the fun is! Maybe think of it more like Jazz; You have to know the rules before you can BREAK them.

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Mikel Z

[#Pedro]

Hehehe, I love Jazz!!!
And this is exactly what I did with the impulse generator:
I studied the standard applications of the cmos circuit 7555 but did not use it. I wanted a forced 0 dB output with any possible load.
So I built an oscillator in a totally different way, with "bad" frequency performance but constant output current and wide supply voltage range (2,5...18 V) without any adaption needed. The circuit is totally against the rules, the 7555 had been designed for. But not against the electrotechnic rules, if not it wouldn?t work anyway.
I did this, because I found out, that standard applications won?t work with modern miniature size impulse heads in pro tape recorders.

Interrupting a DC source with the flash contact and limiting the output level/impendance will always create a rectangular wave form (or spikes) with 24 Hz. When resolving the audio afterwards, maybe with software or controlling the speed of any projector or editing equipment using that signal, you basicly do the following:
You must take advantage of EVERY SINGULAR WAVE, because one frame is only represented with ONLY ONE wave.
And here exactly is the problem concerning reliability: Only one wave missing or one additional wave, created easyly by interferences or not perfect contacts, you make an error of one complete frame.
When you spent only a little bit more effort into the interface circuit, you could create an audio signal with about 10...20 waves per frame, then a pause, afterwards the other multi-wave-burst for the next frame. You increase the reliability 10 to 20 times, with nearly no additional effort, only the 50 cent for the 7555 circuit! And you have a direct "beep" output from your recorder, that can be used directly to control any frame pulse based editing equippment, or handeled with special audio resolving software. A "cnack" can be created by any interference or contact problem very easyly an won?t be able to distinguish from the flash contact cnack, while a "beep" never appears by accidence.

Pedro

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