Laptop technology has been moving forward at an exponential rate in recent years, but for some reason, the technology driving the sound cards in these machines has lagged way behind. Whilst, manufacturers have packed their products with CPUs that have blistering clock-speeds and amazing display technologies, they more than often than not include a very basic sound card. And although this is changing, a good deal of laptops still ship with rather inferior sound playback and recording technology. That said, there are now plenty of products at the upper end of the price range in both PC and Macintosh flavours which offer you a viable sound recording platform for your filmmaking endeavours.
Some things to consider before making a decision to record sound for your film on a laptop:
Sync
Perhaps the most important consideration is how you are going to sync your images with the audio once you get to the edit suite. Most people opt for the traditional clapper route, however if you have a little more money available (or good connections), you may wish to consider picking up a timecode slate. Either way, you need something which you can use as a sync reference point when it comes to getting your audio and video synced up in post.
Battery Life
You would have though it pretty obvious, but you’d be surprised at how many filmmakers forget how a laptop’s battery life is important to the decision. Recording sound on a laptop can be quite power-hungry, particularly if your mic is sucking phantom power from the machine as well. Most standard laptop batteries only give you 4-5 hours of run-time, yet many location shoots can run up to 12-13 hours. Make sure you have enough battery power available to cover you (obviously anywhere you shoot where there is mains power isn’t an issue).
Sound Card Quality
As mentioned in the introduction to this answer, sound card quality in your laptop is an important consideration. Do not record your sound on an old or entry-level machine. Select a laptop which has a quality sound card inside. Otherwise you’ll be much better off recording sound to your camera, than you will be recording sound on a laptop with a sub-standard sound card.
Recording Software
In addition to the hardware, you’ll also need a half-decent sound recording application which gives you enough control to do the job, but isn’t so complicated that it adds to the difficulty of making the recording (particularly when you a jammed into a tiny corner of the set to avoid being in shot).
Microphone Interface
Virtually no off-the-shelf laptops come with proper mic inputs. Most simply have a 1/8th inch jacks for line-in, mic-in and so on, but most decent mics will use standard XLR inputs. A load of manufactures produce Firewire and USB-powered external mic interfaces, so do your research and decide what best suits your needs and available equipment.
Editing Time
A final consideration is of course the additional editing time. When you record your sound in camera, you obviously don’t have to worry about syncing it all up in post-production. When you record sound on a separate device (such as a laptop), there will be a significant amount of time required to go through and sync your audio to your images before you can start editing. In some cases, particularly if your production is under time-pressure, it may be simpler to just record the audio in-camera and save the editing time.