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Recording Audio on a Macbook Pro

Discussion in 'Sound, Music, and Intercom' started by Hughesie, Aug 11, 2008.

  1. Hughesie

    Hughesie Well-Known Member

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    Occupation:
    Freelance Lighting Programmer/grandMA Trainer
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    Today i was recording something on my macbook pro via an external microphone and i was having huge hum issues, i checked the mic and found it was fine then started messing around with the computer and discovered that due to the metal case a large amount of voltage goes through the laptop and the issue can be easily remedied by removing the power cable when doing your recording.

    Hope it helps
     
  2. The_Guest

    The_Guest Senior Team Emeritus Premium Member

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    Are you playing back the audio through your macbook pro's internal soundcard?

    I've found a lot of times that with almost any laptop (regardless if you're using a mic or not), if you use the internal soundcard and run it through a powered speaker or PA while the laptop and powered speaker are drawing from the same power source, buzzing almost always occurs. There is something about computers mixing with analog audio using the same power is always a recipe for disaster (particularly w/ residential power). Ever have a brilliant light board operator plug their console into your rig's buzz-free power?

    I have a macbook pro and I always run all my audio through my firewire audio interface (Motu 896HD). This is how I always do it and I never have problems. I have fed my laptop straight into consoles w/o the interface and have not had problems (maybe the power distros have good isolation).

    It sounds like you have some sort of ground looping problem. Are you running an unbalanced mic?
     
  3. mixmaster

    mixmaster Active Member

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    I had a similar problem with my old Gateway. Anytime we recorded from an external source with the laptop plugged into wall power, it buzzed like a swarm of angry bees. Assuming that it was a ground loop, I proceeded to lift the ground on the line feeding the laptop, but it did not fix the problem. I had this same issue in multiple venues so then I knew it wasn't dirty power or a bad ground. I tested it with multiple sources, mics, mixers, headphone jacks, etc. I had also tried it with balanced and unbalanced mics so I know it wasn't a balancing issue. I could record on battery and it was clean as a whistle, but the battery was getting old and was just barely making it thru a show. When I discussed this with the school's IT department, I was told that they had received multiple complaints about this and had determined that on this particular model of computer, the sound card and the power supply were too close together and AC present in the Power supply when the laptop was plugged in was creating a buzz on the sound card circuits. Since batteries are DC there was no buzz when operating on battery. While I was a bit skeptical, they upgraded my laptop to the newer model and the problem went away. Since then I have put an old laptop and a new laptop right next to each other, the old one buzzed, the new one didn't. I guess the new laptop's power supply is across the board from the sound card. I don't know if this could be your problem but it may be something to consider. Using firewire or USB would also solve the problem, because the digital stream wouldn't be affected by the AC bleed, but reqires the expense of an external firewire or USB source.
    Matt
     
  4. cdub260

    cdub260 CBMod CB Mods

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    There is a problem with your IT department's diagnosis of the problem, though the fix clearly worked. Laptops have an external transformer/power inverter which converts the AC line voltage to a usable DC voltage for powering the computer and charging the battery. Its the large box shaped thing built into the power cord. It's far more likely that the transformer/power inverter was generating some type of harmonic in the DC going into the computer, which was then picked up by your sound card.

    Anyone with more knowledge of these systems, feel free to correct me if I'm wrong.
     
  5. mixmaster

    mixmaster Active Member

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    Your right, I never thought about the AC never making it to the laptop. Straight DC wouldn't induce a hum.... but the same software in the same situation, the only change was the new model laptop. Multiple units of both models with multiple power cords showed consistent results. Would that suggest a design flaw in the old model power supply? I might have to get one of the old adapters and hook it up to an oscilloscope.....
     
  6. maxkelley

    maxkelley Member

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    Perhaps, since the power supply for the laptop is likely a switching power supply, the switching frequency is leaking into the power source to the laptop due to bad filtering in the power supply, and therefore into the audio chain, and voila, hum!

    In both radio (such as ham radio) and audio fields, transformer-based power supplies are preferred over the noisier switch-mode power supplies.

    Just an idea.
     
  7. Chris15

    Chris15 CBMod CB Mods Premium Member

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    With modern switchmode designs and improved filtering, you will now find Switchmode power supplies in some of the world's most highly regarded amplifiers... Or perhaps our muscles are colouring our judgement after lifting one of each...
     
  8. maxkelley

    maxkelley Member

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    That's true, they have become cleaner, but those audiophile types still like to break their backs :) Isn't it also possible that to save space or $$ or something, you can still find switch-mode power supplies that are sorta dirty?
     
  9. waynehoskins

    waynehoskins Active Member

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    In radio it's chiefly because we're concerned with more than 20Hz-20K. With audio, that's all of the spectrum a power supply has to be clean in. In radio, it's got to be clean up to 30 megs where we're looking for very weak signals (50 uV = S9). That's harder to do with a switcher, so we use the good old iron there.
     

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