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SN65HVD3088E: for transmitting digital audio

Trevor brearley
Prodigy 50 points
Part Number: SN65HVD3088E
Other Parts Discussed in Thread: THVD1550

Hi,

I'm a bit of a DIY novice, but I was wondering if I could use the SN65HVD3088E to transmit SPDIF (digital audio) at 192khz 24bit data over a twisted pair in a cat5 cable and then re-establish a single ended SPDIF signal at the other end? Ie one as a transmitter and the other as a receiver.

My concern is that the original SPDIF signal is only 0.2-0.6v, so would it 'trigger the transmission threshold' (if you get my meaning)?

If it would work, I'd then like to 'split' the original twisted pair into two equal ones (Left and Right signals) and feed it in to 2 SN65HDV3088E receivers - so the stub (that is referred to at the end of the data sheet) is not really a stub at all.

Would this require equal termination resistors of twice the normal value so the transmitter still see them in parallel and therefore the correct impedance (I'm guess about 100 ohm, so that would be 200 ohm at the receiving end of each twisted pair).

Many thanks,
Trevor

  • 0
    TI__Mastermind 25705 points

    Trevor,

    Yes, you need to level shift SPDIF signal to TTL signal. After the signal is fed to SN65HVD3088E, it becomes differential RS485 signaling. The twisted pair is recommended to be terminated at both end for signal integrity. The receiver converts the differential signal back to be single-ended. Then you may want to level shift it back to SPDIF. Please let me know it makes sense to you.

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 50 points

    Hi thanks for the reply.

    So, even thought I'll only be using these with one being dedicated as a transmitter (there will never be bi-directional transmission) is there is still a requirement for a resistor across the twisted pair at the transmitter? - I would have expected a series resistor of 50 ohm (I guess this value) in series with the two outputs of the transmitter.

    I can understand the need for identical 100 ohm resistors across a twisted pair where bi-directional data is expected.

    Back to the query of two receivers at the end of two twisted pairs from a single transmitter, what value of termination resistor across the two twisted pairs would be required?

    I'm trying to attach a PDF showing my understanding and my proposed application/solution. BUT the PDF seems to look like a folder icon - I'm hoping it will download when you get this. Failing that I could reply via my normal email (thbrearley@yahoo.co.uk)

    May be I've chosen the wrong product ... I'd like the device/devices to be able to work from a single +5v supply, so could you suggest a solution that would be cheap!


    Thanks,
    Trevor

    SN65HVD3088E.pdf

  • 0 in reply to Trevor brearley
    TI__Guru 62920 points
    Hi Trevor,

    The attachment worked. Hao is on vacation today, so I'll try to help out.

    For a unidirectional (simplex) point-to-point link, termination only at the far end is OK. You typically wouldn't use the series 50-Ohm resistances at all since they will attenuate the signal.

    When driving two receivers, you would be able to place termination at each one without overloading the driver. Typically the termination resistance value is set to be equal to the characteristic impedance of the cabling, although the values could be increased somewhat if driver loading is a concern. (Transmitters will typically either spec a maximum load current, a minimum load resistance, or a minimum output voltage at a particular resistance condition, and it is best to remain within these guidelines.) As the degree of mismatches between these impedances increases, though, the portion of the incident signal that gets reflected back towards the transmitter increases as well. In many cases this is OK (for example, for small reflections or if the propagation delay through the cabling is much faster than the signal transition times), but in more extreme cases it can degrade signal integrity enough to produce bit errors.

    In terms of low-cost differential driver devices, the SN65HVD3088E is a good choice for higher data rates. (Based on your first post, it sounds like you need to transmit about 5 Mbps of data - let me know if this is incorrect.) If you would like a transceiver that is more robust to transients (or are currently using an external TVS diode for protection), you could look at the pin-to-pin THVD1550 as well (which integrates higher ESD protection).

    Max