PLEASE HELP - AV transmitter hiss

hi there -

this is my first time here so please be gentle.

i am building some bits and pieces for my video filming work including a 2.4ghz video/audio (stereo) transmitter/receiver which works fine so far.

however, when all plugged up and working, i always get a slight hiss on the sound from the receiver. when i unplug the video cable on the transmitter (i.e. to no longer transmit a video signal but still the audio), the hiss goes completely.

am i missing something here or should i be adding another component that will sort this?

many thanks in advance,
nick.

actually it's more of a hum and seems to vary in strength depending on what the video signal is:

very bright white saturated video signal produces a loud hum.
dark does the opposite.

any help would be most appreciated.

Sounds to me like you have a "ground loop" problem. This can happen between various pieces of gear that have different grounding schemes internally.

What may be happening is that one piece of gear is causing a small current to flow through the grounded shields of your cables, and is being picked up an amplified. Check the grounding in the video transmitter, and see if adequate power supply isolation and decoupling is there.

The relationship between the video content and the noise may be related to the signal to noise ratio of white and black objects in the video signal. If I remember correctly, white video signals are lower in level than black, so any noise in the video would be far more apparent in a white signal.

Nigel can probably shed more intelligence on this, but I think that is how it goes.

zevon8 said:

The relationship between the video content and the noise may be related to the signal to noise ratio of white and black objects in the video signal. If I remember correctly, white video signals are lower in level than black, so any noise in the video would be far more apparent in a white signal.

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White level is high, and black level is low, with the sync pulses being below black level (blacker than black?).

so should i be using a 1:1 isolator to sort this problem? if so, would i need to use one on each signal line: video, audio left &audio right...

also, i'm a bit worried about the isolator reducing the signal strength. do you guys have any tips on what to look out for.

bmdesigns said:

so should i be using a 1:1 isolator to sort this problem? if so, would i need to use one on each signal line: video, audio left &audio right...

Click to expand...

I don't see any reason why that would help?, but it's totally impossible to offer advice as we don't have the faintest idea how your circuit is designed or constructed - it could be a design error, or a constructional error - or both?.

You posted this question in two forums, and I asked in the other one how you were actually doing the sound - I've never seen an answer yet?.

Nigel Goodwin said:

White level is high, and black level is low, with the sync pulses being below black level (blacker than black?).

Click to expand...

That's it, mybad. It has ben awhile since I worked on a video amp circuit. I must have been thinking of the AGC overdriving the higher signal and bleeding into the sound circuits. It used to be quite common if I remember correctly, especially on cheap poorly aligned sets, a quick adjustment of a coil and you were good to go.

i'm sorry for not being clearer in my earlier posts - i did not realise that this would aggravate people.

my knowledge in electronics is limited so i will try to explain where i am at:

the transmitter and receiver parts are pulled from a consumer version very similar to the one seen on the Rapid Electronics website (part no. 43-0308).

http://www.rapidelectronics.co.uk

the transmitter side is left pretty much untouched (although will eventually will be coupled with a 7" tft monitor). the receiver side is connected with a dual L386-amp circuit that allows me to monitor the incoming audio on a pair of headphones. this amp circuit works fine and i have made sure to keep the input grounds seperate from the output grounds to avoid oscillation.

in both the transmitter and receiver circuits, the video and audio channels share a common ground with the power supplies (both regulated 12V dc through mains adaptors). this is how they were in the original consumer version so i have kept it that way.

like i said, the hissy-hum dissappears when i pull out the video cable in the transmitter and varies depending on the image make-up (bright versus dark areas). also, if i were to pull the video cable out of the receiver, an extremely loud buzz is heard - i'm not sure if this is because i need termination...?

like i said, i don't know much but i am trying to learn and any help that you might be able to offer me would be most appreciated.

To save cost, the receiver probably demodulates the audio after passing it through the video IF amplifier instead of having its own mixer and IF amplifier like an FM radio. Then when the video gets too white its carrier stops and the audio demodulator doesn't have any signal anymore for those moments. The buzz is the audio demodulator turning on again at the blacker than black vertical sync frequency when the video carrier is at full power.
The transmitter should limit the white to no less than 10% modulation or have a receiver's FM audio demodulator with better AM rejection, or both.
Maybe something in it needs adjustment. :?:

You get what you pay for. A point is reached in circuit design where the circuit works adequately and is inexpensive. Reduce the cost only 5% and it performs terribly. Some get-rich-quick clowns remove some important parts to make it cheaper, since they didn't affect the video very much (and the audio was turned off for the test). :cry:

audioguru said:

To save cost, the receiver probably demodulates the audio after passing it through the video IF amplifier instead of having its own mixer and IF amplifier like an FM radio. Then when the video gets too white its carrier stops and the audio demodulator doesn't have any signal anymore for those moments. The buzz is the audio demodulator turning on again at the blacker than black vertical sync frequency when the video carrier is at full power.

Click to expand...

This is why I've asked (twice now!) how the sound is transmitted, he also claims the sound is stereo?, which introduces further complications!. But as we've no idea how it's been done, it's impossible to make informed suggestions.

BTW, the sound buzzing you discribe is know as "intercarrier buzz" or "caption buzz", due it's likelyhood during bright white captions on screen.