ISSUE:
Using a MacBook Pro connected into an AV system with sound reinforcement and video projector we have a loud background hum introduced into the audio reproduction. The trick? This only happens if the external monitor connection is plugged in. The only thing that stops the hum is removing the external display connection.
This is not the usual laptop issue of noisy laptop power supply as the hum exists with the laptop running from battery, even using a firewire sound card (FCA202) to eliminate audio grounding issues, we still have the same issues.
Oh great, just what we need …
A quick Google of macbook pro audio hum external display
The external audio hum is apparently a known issue when connecting a Macbook, Macbook Pro, or Macbook Air to an external TV monitor using the DVI port…
This would appear to be a pretty systematic problem with the segregation between the audio and video system in the MacBook Pro. Two solutions appear to be popularly in discussions;
Solution 1: Audio Ground Loop Isolation
Using a Ground Loop Isolator in the audio connection (eg. Jaycar: Car Stereo Line Isolation Transformer $12.75). [Update: 22-AUG-2012 AA3085 has been replaced with AA3084 $9.95]
_*If you’ve isolated the issue to a hum when you connect your DVI port, the only viable and successful workaround is to buy a ground loop isolator for your audio cabling.*_
– Audio hum w/external display? [discussions.apple.com]
It turned out that you need to get an “Ground loop isolator” which is a box that quote “works by breaking the earth between your computer or audio components and stereo so there’s no path for current from an electrical imbalance to flow”.
– audio noise when using DVI out [discussions.apple.com]
The line filter can be installed at the wall plate and left with the AV drop cable as a part of the rooms AV infrastructure. As the installed infrastructure uses 3.5mm jacks the line filter will need RCA to 3.5mmm adapters.
Solution 2: Digital Audio Out
The MacBook Pro allows for Digital Audio Out and In via the following ports;
Audio line in/optical digital audio in port
Connect your MacBook Pro (15-inch Core 2 Duo) to a line-level microphone or optical digital audio equipment. The Audio In port accommodates both optical digital audio input and analog audio input.
Headphone/optical digital audio out port Connect external speakers, headphones, or optical digital audio equipment. The headphone / line output jack accommodates optical digital audio output, analog audio output with a 24-bit, 44.1-192 kHz D/A converter, digital audio output up to 24-bit stereo and 44.1-192 kHz sampling rate and supporting encoded digital audio output (AC3 and DTS). For analog headphone / line output a standard audio cable with 3.5mm metal plug should be used. For digital audio, a standard Toslink cable with a Toslink mini-plug adapter can be used.
So using a Toslink to Mini Toslink(3.5mm) Cable# and a Digital to Analogue audio converter (eg. Jaycar: Digitech Digital to Analogue Audio Converter AC-1603 $69.95) we can attach out digital audio into the sound system. You also require a RCA to 3.5mm converter to finish the rig.
(# For our test rig I used a standard 05.m Toslink cable WQ7261 $16.95 and adapted one end using a Mini Toslink adapter PA3574 $5.95.)
The digital to analog system requires power, and needs to be located at the laptop end of the system. This system is perhaps more suited to a traveling user who requires a very clean signal.
This solution does NOT clear the majority of the hum from the audio signal.
Which Solution?
Given the price difference and the suitable result from using the line filter, I would recommend the line filter for most situations.
NOTE: Originally written 2011/07/27. Published 2012/08/16
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