Sound Suggestions -
How to Get the Best Sound When Shooting Direct to Digital Video
, by Larry Loewinger
Has the computer revolution really improved out lives? We can argue this point endlessly as change itself endlessly accelerates. But there is one corner of this vast revolution that has truly altered the production landscape for the better, and that's the introduction of digital video (DV) and its cameras.
It is not that the images these cameras produce is either of broadcast quality or like film (though they are surprisingly good). Nor is that the sound they record is any better than what DAT tape recorders and postproduction editing systems can reproduce (though this, too, is still of a surprisingly high order). What is really striking about DV cameras is the contrast between their pea size and price and the quality they deliver.
This is true in sound as well as picture- if you know the peculiarities and points of difficulty that are unique to digital cameras. To know where the hazards lie is the first step to better sound recording, Interestingly, the story of doing sound for digital video is not what you would expect--about microphones, mixers, fishpoles, radio microphones, and the other accoutrements of sound recording. Rather it's the story of the digital camera's themselves-their audio inputs, meters, and recording capabilities. We'll focus on two cameras, Sony's DCRX1000 and its replacement, the DCRX2000 , and Canon's XL-1 www.canondv.com/xl1/index2.html. Canon and Sony both offer other digital cameras, but these two have come to dominate the digital video prosumer market.
Consumer, prosumer, and professional are three words that have important practical meaning for us. They refer to the design, appearance, and ultimately the expectations of what these cameras can do for us. From an audio point of view, the differences lie in the input or entry point of sound: whether the two channels of audio signal can be manipulated manually and independently of each other, and whether there is time code signal applied to the tape as the audio is being recorded.
A consumer camera usually has an unbalanced input with a stereo mini plug as its input connector, automatic gain, and no time code. Fully professional cameras are bigger, offer balanced audio through XLR connectors, and output and record with a time code sync reference. Between them is the prosumer model which generally has a mini plug as is input connector but also has switchable manual/automatic gain and sometimes a variant of time code as a sync reference. The digital video cameras being discussed here occupy this middle, prosumer range.
The stereo mini plug may be the worst audio connector ever designed for any thing but home stereo applications. Yet it is the audio entry point of almost every consumer and prosumer DV camera. Mechanically fragile, it rarely makes a proper, snug fit into the camera. This makes it susceptible to hum and to audio drop outs. It provides minimal strain relief to its attached cable. If you are moving around as in a documentary situation, the stereo mini plug connection can cause you no end of problems. Worst of all, the signal that flows through it is unbalanced.
Balanced, unbalanced, why should you care? A balanced audio signal, one that is independent of ground, can travel long distances without quality loss, and is relatively impervious to hum. Another term for balanced is electrical floating- above or below ground. A balanced audio line runs within a three-wire cable: two audio leads and a hum-resistant shield which is usually grounded. Virtually all of the microphones you will use, whether condenser of dynamic, shotgun a lavalier, radio or hard wire, have balanced outputs. So far, most of the prosumer DV cameras have unbalanced, mini plug inputs. The mating that occurs at the DV camera's audio input is the single most vulnerable connection in your audio chain. It has spawned a tiny cottage industry of adapter boxes that convert your audio signal, no matter what its level, into signal the camera can accept.
"These adapter boxes," says Steve Robinson, a practiced sound engineer who has specialized in audio for video, "accomplish three important things. They include transformers which drop the input impedance, two volume controls to manipulate the left and right channels, and most importantly, a technique to create a ground connection between your external audio and the camera." At the outset there were two brands of adapter boxes-Beachtek and XLR Pro. (Canon makes its own XLR audio adapter, the MA-100, for the XL-1 camera) Beachtek, which appears to be the only one of these two still being sold, establishes ground by screwing into the bottom of the camera. The base plates of the Canon and Sony cameras are different. Consequently, you have to match your Beachtek adapter to the camera you use. Also, as Robinson notes, "If the camera goes on a tripod with a quick release plate, which is attached to the adapter box and the adapter box which is attached to the camera, the ground connection can slip." If this situation occurs, he recommends the generic fix--gaffer's tape.
The DV audio standard is two channels at 16 bit audio or four channels at 12 bit audio. Sony's VX1000 can only record two channels in he camera, but it does leave room for the two additional channels to be added in postproduction. The Canon Xl-1 does record all four tracks. The notion of recording onto four separate channels or tracks sounds great, even at the 12 bit reduced rate which is okay for dialog recording. In the abstract it allows you to postpone your audio decisions and generate more flexibility for the mix. In practice though, it may create more headaches than it's worth. The first hint is the design of your audio adapter boxes. Notice, two inputs not four. But the main reason for this, as Robinson notes, is that some of the consumer editing software and hardware that is now commonly used can't handle four audio tracks. Even though Sony introduced four track recording in their Betacam system, it has never been very popular and nor is it likely to be in DV.
If you're a one man band following your documentary subject around, you are likely to be using the camera mic or something equivalent, such as the Sennheiser ME-66 short shot gun mic, and possibly a radio microphone on your subject. (See, for example, Steve Bogar's suggested rig on pages 29-31.) As outboard mics, they are connected through the Beachtek or XLR Pro adaptor boxes. You are monitoring your sound through headsets and adjusting the gain of these mics at the adapter.
If your production has a bit of a budget, you might have hired a cameraperson and a sound recordist who operates with a mixer. The sound recordist can hard wire his or her audio to the deck through an audio breakaway cable or else broadcast it wireless to the digital camera. The latter option sounds neat, but it raises a whole host of questions. Hard wiring through a breakaway cable--one that can be quickly split at one point near the camera--is the safer, preferable option. It gives you reasonable assurance that the signal you hear is the one being recorded onto the deck, provided you have set up a proper tone ratio between the mixer and the meter on the camera, and that, through the breakaway cable, you are monitoring the audio that is looping through the video deck. This works in any controlled situation where the camera is stationary.
In any run around situation, whether it be dramatic or documentary, the question remains how to deliver the sound from a mixer to the camera. In many instances an audio breakaway cable can still work adequately, but we live in a world of convenience in which the pressure to go wireless is intense. Radio microphone technology has improved exponentially over the last decade. We have advanced from first generation radio microphones operating in the very high frequency (VHF) range that provide a fragile link between subject/actor and sound person, to ultra high frequency (UHF) diversity microphones that create a link so solid that they are replacing the ubiquitous hard wire connection between boom operator and sound mixer on feature sets. In other words, radio technology is beginning to replace the microphone cable wherever it is used in a film or video shoot. Is this radio mic technology appropriate for the DV filmmaker? The answer is a hedged "Yes, butó." There is a new generation of inexpensive radio mics ($600-$1,200) offered by the major manufacturers, among them Audio Technicia www.audiotechnicia.com/guide.wireless.index.html, Lectrosonics www.lectrosonics.com/wireless/wireless.htm, and Sennheiser www.sennheiser.com/rf_wireless/hf_1b001.htm, that cater to the digital video maker and do the job well. Are they of sufficient quality and reliability to establish a solid wireless link between the mixed sound source and the camera? It depends on how much of a risk taker you are, since you often forego the opportunity to monitor the sound you are capturing.
"It becomes a question: Do you have a wireless return for headphone monitoring of what you have actually sent?," Robinson says. "I have personally chosen to spend the money on good wirelesses and not worry about an audio return when we are running around. Do I give the cameraperson headphones? Yes. Do they wear them? It depends on the cameraperson."
Serious wireless units start in the area of $2,000 and go up from there. An investment in professional wireless technology can quickly escalate to $10,000, and way beyond. Then you are left with the question Robinson raises. Who monitors the audio and how? One answer is more radio technology, like wireless headphones. A company called Comtek makes the industry standard wireless headphones system. But a wireless feed between the sound mixer and the camera and a wireless headphone return to the sound person (and anyone else, for that matter) involves more money, weight, complexity, and, most of all, more risk.
There are three final issues, simple and practical, which may sound mundane but are rather important. First, Robinson recommends using right-angle mini plugs, phone jacks, and even XLR connectors into and out of the DV camera. Right angle connectors act as strain relief on your cables. They shrink the profile of your camera and reduce the risk of breakage on location.
Second, understand the difference between 0 dB on your mixer's VU meter, or 0 VU as it is sometimes referred to, and 0 dB on your camera's peak meter. There are two issues here: All digital audio is monitored with peak reading meters that measure the true range of your audio signal. As technology evolved, zero in the analog world became merely a warning- a serious one!- that if you didn't control your gain, trouble was close hand. In the digital domain, zero is a cliff beyond which you have tumbled into digital distortion which sounds like clicks. So tone at 0 VU on your mixer is places somewhere between- 20dB and -12 dB on the camera meter than on a VU meter.
Third, listen to your sound as often as you can from the audio return coming from the headphone jack of the DV camera. If you are monitoring from a mixer, Robinson advises using good quality headsets that surround and enclose your ears (circumaural). If you are working alone with the camera on your shoulder, you may want to wear open-air headsets (supra-aural) that keep you aware of your immediate surroundings. The cameraperson may prefer to listen with headsets that go directly into the ear (inter-aural). Beyer www.beyerdynamin.com, Sennheiser, and Sony are the primary headphone manufacturers. Whichever headsets you use, listen closely both to the content and the quality of the sound you are recording. Listening is the key to great sound recording. Proficiency in sound starts with a happy combination of deft technique and good technology, but it ends with what your ears hear and your brain processes.
Larry Loewinger is an audio engineer and documentary producer.