IR transmit/receive - oscilloscope trace

[RMIM]

Dear all,

I’m working on a new ir project (trying to send out an ir signal to turn my projector on/off) using a pic.

First step was to try and produce a carrier signal 38khz then modulate this.
I hooked up my efforts to my picoscope and did not see what I was expecting.
First my carrier bursts seem to precede the output of my Vishay tsop 4838. I expected them to be in sysc.

Then if you measure the duration of the burst (in red) it’s about 670us (as I tried to achieve in software) but the corresponding response (in blue) is about 720us.
Is this normal? Am I not using my scope properly? I don’t mind if they are not in sync but if the width is not the same, the tv/projector etc is not going to react.

Any help appreciated.

 

[jpanhalt]

That looks about right to me. Go to figure 7 of the datasheet. The detection (td) is delayed 7 to 15 cycles. The duration of low (Tpo) is also delayed.

Remember, the noise canceling algorithms in the detector have to see something definite before responding. Notice that ranges are given for the delays and extensions.

John

 

[RMIM]

That looks about right to me. Go to figure 7 of the datasheet. The detection (td) is delayed 7 to 15 cycles. The duration of low (Tpo) is also delayed.

Remember, the noise canceling algorithms in the detector have to see something definite before responding. Notice that ranges are given for the delays and extensions.

John

Thanks for that John. I did not know about the delay.

But does that mean I should modify my carrier bursts (make them shorter in this case) so the output from my IR Receiver is low for the correct number of microseconds?

Have far out +/- microseconds can one be before the tv/projector etc will not respond and see it as a different signal?

 

[jpanhalt]

For the first question, your receiver is probably not very sensitive to the duration of the data bursts . Just study the datasheet, and particularly Figure 7 and the one below it.

For the second question, the answer is pretty much the same as for the first. Your receiver probably is not very sensitive to the duration of the data transitions. Variations in timing are the norm, thus methods of encoding have been developed that are are quite insensitive to that variable. Here are some links. The first is a general discussion. The next 3 are for Manchester coding specifically. Basically, one looks at transitions, not durations.

General concepts:
http://www.rhyshaden.com/encoding.htm

Manchester:
http://www.embedded.com/design/conf...ack-to-the-future-Manchester-encoding--Part-1
http://www.embedded.com/design/mcus...ack-to-the-future-Manchester-Encoding--Part-2
**broken link removed**

John

 

[Nigel Goodwin]

Dear all,

I’m working on a new ir project (trying to send out an ir signal to turn my projector on/off) using a pic.

First step was to try and produce a carrier signal 38khz then modulate this.
I hooked up my efforts to my picoscope and did not see what I was expecting.
First my carrier bursts seem to precede the output of my Vishay tsop 4838. I expected them to be in sysc.

Then if you measure the duration of the burst (in red) it’s about 670us (as I tried to achieve in software) but the corresponding response (in blue) is about 720us.
Is this normal? Am I not using my scope properly? I don’t mind if they are not in sync but if the width is not the same, the tv/projector etc is not going to react.

You might have a read of my PIC IR tutorial, which explains the Sony SIRC's remote system.

But all work in a similar way - and have to work in the knowledge that the width and pulse timing at the receiver bear only a slight resemblance to the transmitted ones, which is why you can't just send RS232 data. This is why SIRC's uses a widely varying pulse scheme, and many others use Manchester coding (where it's the transitions which carry the data, and not the pulses).

But all schemes are VERY simple to transmit using a PIC and simple software loops - you just have to send the correct number of 38Kz pulses - I've used my basic PIC routines to transmit lot's of different types of IR. Reception is a LOT more difficult though, compared with how trivial transmitting is.

 

[RMIM]

For the first question, your receiver is probably not very sensitive to the duration of the data bursts . Just study the datasheet, and particularly Figure 7 and the one below it.

Thank you very much for the info John.

 

[RMIM]

You might have a read of my PIC IR tutorial, which explains the Sony SIRC's remote system.

Thank you Nigel. I will have a look.