Atlona HD570 distortion / subsonic transients

In my linux pc-based dsp system for crossover and EQ of active loudspeakers, I use a pc to send 6 channels of processed audio over hdmi, with an Atlona AT-HD570 to convert all channels to analog before amplification.  Recently I discovered that this setup can distort badly at moderately high signal levels, in a way that produces large transient subsonic signals which, when amplified, could permanently damage loudspeaker drivers.  My measurements point to a flaw in the HD570.

 Sine Wave Response

I connected my pc’s hdmi output to the Atlona HD570, and output a full-scale 400Hz sine wave on one channel while capturing the HD570’s analog output.   The first 250ms of output looks like this:

sine_out_0db.eps

Strangely, the output has a 15ms rise time, and the sine wave rides on top of a large subsonic transient that takes more than 250ms to die out (the black curve is the same analog output, but low-passed at 20Hz).  It seems the HD570 has an under-damped low-frequency resonance in its transient response, at least when it’s driven to high levels.

Attenuating the input by 6dB excites this transient somewhat less but it’s still really bad:

sine_out_6db.eps

The steady-state level is only 3dB (not 6) down from the the full-range case so there is some compression/limiting going on, though it doesn’t look like clipping.

At -12dBFS the transient is much reduced in amplitude, and the steady-state level drops 6dB as it should:

sine_out_12db.eps

But a 20ms subsonic transient response is still evident; it doesn’t really disappear until I drop the input level below -15dBFS.

Response to a Pulsed Sine

To test whether the distortion originates in the pc or in the HD570 itself, I made a test signal consisting of a full-scale 400Hz sine wave pulsed 4x per second with 10% duty cycle; it looks like this:

sine_pulse.eps

I captured the analog output of the HD570 while feeding it this test signal over hdmi from the pc.  I then did the same, but feeding the signal from a Sony BDP-BX120 media player instead of the pc.

With the pc as source the HD570’s steady-state response to the pulsed tone looks like this:

Every pulse has a slow rise time, and initiates a subsonic transient that doesn’t damp out before the next pulse. Exactly the same thing happens with the Sony player as the source:

(the vertical scale is different because I changed the pre-amp level on my recording device).

With identical results from independent sources, I’m led to believe the distortion is caused by a serious flaw (feature?) in the HD570.

Update (18/8/2016): The engineers at Atlona have confirmed this issue on other units.  No word on a possible fix.  I recommend strongly against using the HD570 for any input signals above -15dBFS.

5 thoughts on “Atlona HD570 distortion / subsonic transients

  1. Hi Richard,

    I had a similar problem when using HDMI via Foobar with the crossover plugin. I tried this with 2 different HDMI units, an Onkyo SR706 receiver and a JBL M2 preamp with the same results. I had audible bass distortion on some, but not all music. Some examples; Jack Johnson – On And On, Yellowjackets – Dreamland. When I switched from HDMI to SPDIF there was no distortion. Great post by the way.

    • Interesting, and strange. Makes me wonder if it’s an issue with the upstream hdmi encoding, not the decoder itself. And yet I got identical results when playing the same .wav file via two different hdmi sources.

      • I forgot to add that I didn’t use an HDMI converter like you did, I ran HDMI from an MSI E350IS mITX PC straight into the receiver. I also have a Denon DN-A7100 preamp that I’d like to try and see if the same thing happens. I really wanted this to work because I think it’s one of the best ways to do it.

        Thanks,
        KC

  2. I was getting a lot of subsonic information in my setup, which caused the woofers to flap around. I solved that with a 30hz 4th order high pass crossover, now no more flapping. Another thing that I’ve noticed is that you can get distortion on the signal any time the total signal is above 0db. Are you going above 0db with any of your filters?

    • Nope, I’m careful to keep all my levels below 0dB. In my case the culprit was definitely the Atlona converter: the Atlona engineers have confirmed the issue. A 30hz high pass does reduce woofer excursion, but it can’t eliminate the higher-order distortion products that the Atlona unit generates.

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