Bad Audio over HDMI Output of MBP

I have a problem happening with two WC Pro machines running the same project file and similar hardware.  I'm running two MacBook Pros (will list full specs below) taking the HDMI out into a Video Devices PIX-E7 monitor/recorder.  The audio sounds like there's a ping pong delay and also distorted or "bit crushed".  It doesn't sounds this way all time time, it comes and goes.

I've gone through the audio setting on the macs and WC.  In WC, in preferences, general, audio interface, I'm set for HDMI.  On the MBP, under System Preferences, Sound, Output, I'm set for HDMI.

With I connect the HDMI out of the MBP to the PIX-E7, I get the wallpaper and system sound of the MBP - this is even before I have WC open.  I open WC, and select "External Display Output", then choose the PIX-E7, the program out video of WC shows up on the monitor.  Unless I have the MBP set for HDMI audio out, and sending audio out in the preferences of WC, I won't hear the audio.

Sometimes it's fine, but it seems to be stressed when I switch video sources, add/remove lower 3rd titles - meanwhile I hear the ping-pong audio and distorted sound.  I hear this when I monitor the headphone out of the PIX-E7, or if I which the audio out of the Mac to the headphone out.  However, the sound totally clears up when I disconnect the HDMI going to the PIX-E7.

I've tried several HDMI cables with the same result.  I've connected the USB 3.0 converters on the dock, and directly to the MBP with the same results.  I mainly use the dock for the hardwired RJ-45 connection.

Here's the equipment used:

Apple MBP Retina mid-2015, OSX 10.12.1 Radeon R9 M370x, 16GB RAM, 500GB SSD

(2) Epiphan AV.io SDI to USB 3.0 interfaces

(1) ioGear GTD720 Thunderbolt Dock

Video Devices PIX-E7 Monitor Recorder

Wirecast Pro 7.2.0

Let me know if there's any more info about the setup.  I really want to use the PIX recorder for monitoring and recording the program output, but it seems to be the device causing the bad audio when it's connected.  The bad audio can be heard on the monitor, on the MBP, and in the encode, and and recorded archives.



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  • When Wirecast is sending video out External Display Output the audio would be the system sound. In many cases that would be 44.1kHz rather than 48kHz that many devices need. 

    I'd recommend checking in Apple Audio Midi Setup utility. To confirm the issue isn't related to the input device I'd suggest sending the output to a computer monitor with built in speakers.

    Also you may need to test Wirecast > Preferences > General > Audio Interface: 

    One reason Wirecast Pro has Video Out through Blackmagic and AJA devices is that it'll send professional video and audio and avoid the issues that might come from GPU and computer system sound issues.

    You state:


    The bad audio can be heard on the monitor, on the MBP, and in the encode, and and recorded archives.

    It would be odd that External Display Output is impacting any of the above though. Again I'd remove the PIX-E7 from the equation and test going to a computer monitor. First we need to confirm that the issue is specific to the PIX-E7.

  • Hi Craig,

    Thanks for the info above.  Here's what I found:

    I set the MBP's system audio to 48k 16-bit, and confirmed all other devices are set for 48k / 16-bit - HDMI, USB, and AV.io audio devices are all set for 48k 16-bit in Apple Audio Midi Setup utility.

    I found there was a large jump in firmware for the Epiphan SDI AV.io USB adapters, from 2.3 to 3.1. More in this below with screenshot, and info about video processing.

    I connected a 9" JVC monitor w/HDMI and heard the same results, this was before the AV.io firmware updates.

    I opened the same project I've been testing with, which was a town hall webcast I did a few weeks ago. There were two live SDI feeds coming in over the AV.io USB adapters with a mix of full screen and composited shots on the 2nd layer, and lower thirds for presenters (5) on the first layer. Layer 4 was a backdrop .jpeg, Layer 5 was used to bring audio from one of the AV.io adapters for program audio.

    What I see happening now is intermittent audio when I switch between shots or add lower thirds from layer 1.  It's like WC is tripping over the complexity of switching between the shots, and the audio is dropping momentarily. 

    What I think made the biggest difference came from the firmware update from the AV.io interfaces. After the update, I got a choice of input settings in the inspector's System Device Properties. On my webcast a few weeks ago where the audio was really bad, I was coming in 1080i. With the new firmware, I can dial down the input - seen with the attached screenshot. I'm encoding the webcast as 768 x 432 748k, so I set the video setting on each interface to 960 x 540. My CPU usage in WC went from 35-45% to 15-20%, and I've barely heard the audio drop out, but still do occasionally when I switch shots. I've let an a recording go for a couple hours, recording with the 768 x 432 748k preset on a external USB 3 SSD drive. I've let it run for a hour and check in on the audio and will still hear it drop occasionally when I switch.

    Question:  Is it better to drop the input resolution the inspector's System Device Properties or leave it at the incoming resolution?  If I have 1080i coming from a camera, and my webcast encode is 768 x 432, should I drop down the signal in the properties to maybe 960 x 540?  Would the drop in resolution happen in WC, or at the converter?  If I ISO record, would I be recording the dropped resolution version?  Just wondering what best practices are here.

    Here's a couple other unrelated things I've observed in 7.2:

    • I've had the auto live get stuck and not work, and suddenly start working again after doing Command L.
    • I was experimenting with ISO recording a camera.  I had the recording going to a single USB 3.0 SSD external drive, and saw even with ProRes422LT, the recording was only 15fps.  I didn't see any other controls for the recording format other than the PreRes drop down.  Is the 15fps because I'm using a single drive?

    Sorry to go on..  Thanks in advanced so much for the valuable info!


  • It's not clear whether you followed my instructions and removed the PIX-E7. Please do follow my testing instructions.

    The AV.io firmware should only affect that device. It's not clear what role the AV.io is playing in the audio but we must test that separately.

    I want to confirm whether the PIX-E7 contributes to the problem

    To troubleshoot this you need to removed complexity and test each individually. 

    You should start separate topics for separate issues and include specs and steps in each so an engineer can follow without having to cross reference topics.

  • Hi Craig,

    I removed the PIX-E7 and tested both without anything connected to the HDMI, and also with another monitor connected to the HDMI. I have not found any regularly occurring audio issues like I was hearing before. I did notice an improvement in the overall performance of Wirecast after lowering the incoming resolutions of the AV.io interfaces from 1080i to 720p.

    I'll make new topics for the ISO record and auto live if they're still an issue after installing 7.3.  Thanks for your help!

  • Thanks for reporting. You're welcome of course.

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