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Hum on SCART Output of HDR-2000T

Discussion in 'HDR-1800T/HDR-2000T Freeview Recorders' started by GrahamPye, Oct 26, 2016.

  1. GrahamPye

    GrahamPye New Member

    When I connect a TV to my HDR-2000T's SCART output I get a background hum. I've tried more than one SCART cable, and more than one TV, so I know it's coming from the PVR. Do they all do that, or have I got a bad one? Yes, I know that I shouldn't have bought a 2000T as several of the posters here have mentioned..

    And yes, I do need to use the SCART output because although the local TV is connected with HDMI, I need to send the PVR output to all the other TVs, which I do using an RF modulator connected to the SCART output. FWIW the hum is present after traversing through the RF connection to the other TVs too.

    I've tried disconnecting the HDMI cable and only using the SCART output but that doesn't help. I've also tried switching the SCART output between the two Video modes.

    I have a vague theory that the intensity of the hum may be related to the amount of white in the picture so I wondered if the video was somehow interfering and if I made up/obtained a cable that didn't connect all the pins in the SCART cable that might help - although that seems unlikely if the TV is getting composite video (or is the audio carried separately?)

    Thanks

    PS Searching for "hum" in this forum is a bit of a non-starter, so apologies if this has been posted before!
     
  2. grahamlthompson

    grahamlthompson Well-Known Member

    The audio is seperate. Hum is normally caused by multiple earthing causing currents to circulate in the common screens. First thing to try is using a scart cable where all the conductors are individually screened. Some of the cheaper ones don't have individual screening. Alternatively use the yellow red and white composite and audio outputs with seperate RCA phono cables and a scart adaptor at the TV end.

    https://www.amazon.co.uk/CDL-Micro-...&qid=1477506622&sr=8-2&keywords=scart+adapter
     
  3. GrahamPye

    GrahamPye New Member

    Yes, I wondered if it might be a hum loop - perhaps disconnecting the screen at one end of the cable might address that? Although I have tried more than one cable, but perhaps they were all cheap ones without screening!

    The extra complication is that the RF modulator has a rather crowded back panel, and you need a cable with a straight SCART plug to fit next to the RF connector. I've bought a short fairly good looking cable with straight plugs to make this connection, but without taking it apart I can't tell whether it has individually screened cores. It's a fairly thick cable though, which suggests it might be.

    I did consider using the RCA plugs instead, since the modulator also has RCA inputs, but the audio is only mono, whilst the PVR's RCA output has left and right channels separate. I wondered if joining the two audio channels together would result in the HDMI output being similarly joined, but I suspect that they are separately generated. Maybe I'll just try connecting one audio channel and the video channel from the PVR to the modulator and see if that clears the hum.

    The back panel crowding on the modulator probably means that any RCA to SCART adaptor wouldn't fit. If I could find a source for straight SCART plugs I'd make my own cable - I can do soldering! Perhaps I should have bought a different modulator, but its reviews were good in all other respects.

    Thanks.
     
  4. Black Hole

    Black Hole Trick Cyclist

    It's not a very good idea to simply wire two stereo output channels together and hope that gets you a mono signal. It would be better to choose one or the other, or mix them properly (at the very least put a resistor in series with each before joining them).

    The audio signal over HDMI is simply part of the digital data. The analogue audio is created from it, and nothing you do with the analogue outputs (audio or video) will have any effect on it.
     
  5. grahamlthompson

    grahamlthompson Well-Known Member

    The scart audio output is the same it has left and right audio connections. Connecting by RCA phono and a scart adaptor is identical. By the way turn up the box volume to max (Humax boxes usually default to 50%) and reduce the TV volume accordingly. Change the digital audio out on the box TV setup menu to stereo to make HD and SD audio levels the same.
     
  6. EEPhil

    EEPhil Active Member

    I think I have had a problem like this. It is confused by the hum on the SCART input to the TV - so I'm not positive that the Humax at fault.
    Both my TV and the Humax have RCA sockets - and so I connected using those - no hum! (No HDMI input to TV before anyone asks).
    My other Humax (9150T) is connected via SCART and the hum does vary with picture intensity. Not certain whether the hum is from the tv or Humax though.
    This could explain why people call these PVRs Hummy!
    Very good point. I should have mentioned that. My 2000T is turned up to 18 (out of 20) - any higher and I get distortion. The 9150T is turned up as well - but still hums.
     
  7. grahamlthompson

    grahamlthompson Well-Known Member

  8. prpr

    prpr Well-Known Member

    75 Ohms??? That sounds like a great way to sink a load of signal and would result in more gain being needed somewhere, thus making any hum worse.
    I wouldn't suggest anybody tries this with such small value resistors. It's just daft.
     
  9. grahamlthompson

    grahamlthompson Well-Known Member

    It's nothing of the sort, the cable is 75 ohm (or should be), the hum on a triax trilink unit was traced to a lack of termination which should present at 75 ohms. The resulting impedance mismatch was solved by terminating the cable, in the same way you are supposed to terminate unused outputs on splitters and amplifiers.

    http://www.systemsat.co.uk/f-type-c...-6UpElUbUiW5SRt3ffJWixE7nA8SG2P4pthoCE4Tw_wcB

    https://en.wikipedia.org/wiki/Impedance_matching
     
  10. Black Hole

    Black Hole Trick Cyclist

    Impedance matching is pretty much irrelevant at audio frequencies, and you would only do that if the source of the signal is designed for driving transmission lines. The item in your first link is for RF.
     
  11. Trev

    Trev The Dumb One

    Other than it is normal to have a low Z audio source feeding a high Z input.
    But if it has been established that the Triax tri-link unit needs their O/P terminals terminated in 75 ohms to prevent hum, who am I to argue.
     
  12. prpr

    prpr Well-Known Member

    Are we talking about baseband audio here or some modulated signal? What cable are you talking about?

    Impedance matching is pointless at audio frequencies. It used to be done for GPO circuits but that was 600 Ohms. It's never been 75 to my knowledge.
    These days it is low out, high in, as Trev said. 75 is fairly low .
     
  13. Black Hole

    Black Hole Trick Cyclist

    I would like to be pointed to where in that article it mentions the use of impedance matching to counteract hum.

    The purpose of terminating a transmission line (of known characteristic impedance) with a matching impedance is to eliminate reflections. In the case of a unidirectional link, it is only necessary to do this at one end - usually the receiver end because it's easier to match the line impedance there than at the transmitter end (without also attenuating the drive available). Reflections create echo and standing waves, but only when the length of the cable is of the same order of magnitude as the wavelength of the signal (in the cable). This simply doesn't happen with a few feet of cable and an audio signal.

    Now I've looked up this "triax tri-link" thing, I can see it's just a straight-forward UHF modulator with aerial pass-through, with a SCART input. If you think fitting a line terminator is going to suppress hum on the audio side of the circuit, where exactly did you intend to fit it??? The most likely cause, it seems to me, is a crap PSU circuit in the Tri-link injecting hum on the DC rails.
     
  14. grahamlthompson

    grahamlthompson Well-Known Member

    The hum was only when used with a Foxsat-hdr. No hum when connected to other units with analogue outputs.
     
  15. Black Hole

    Black Hole Trick Cyclist

    What has that to do with an RF line terminator?
     
  16. prpr

    prpr Well-Known Member

    I think he knows he's lost the argument.
     
  17. antipodean

    antipodean Member

    GPO telephone lines are also long enough that it is possible to get typical transmission line standing waves at audio frequencies. A guy I worked with had worked in the post office and told a story of how they had a line that went from A to B through a number of telephone exchanges, but couldn't be used because there was a frequency notch slap in the middle of the audio range used. One day they had an opportunity to travel, going to every telephone exchange on the route this particular cable pair took, checking the records for connections to the pair at each exchange. Eventually they found an additional pair going off somewhere else connected to it, that either had been missed by the engineers making out a worksheet when the use of that line changed, or someone hadn't carried out all the instructions on the worksheet. They disconnected this pair, and the frequency response on the line was restored.

    Also the hybrid transformer in the typical telephone that minimized the crosstalk between the microphone and earpiece relied on having a reasonably matched telephone line.
     
  18. prpr

    prpr Well-Known Member

    Ah, an inadvertent stub notch filter.
     
  19. antipodean

    antipodean Member

    Exactly.