(The following is an excerpt from the juicedLink blog.)
Finally, an audio solution for DSLR cameras ... that works!
Unfortunately, many DLSR cameras do not have the features needed for great audio [meter, headphone, manual override of Automatic Gain Control (AGC)].
DN101 DSLR Audio Accessory
The DN101 attaches to the side of any of the standard juicedLink low-noise preamps (CX211, CX231, CX431). In the photo above, you see a prototype of the DN101 bolted to the side of a juicedLink CX231. Then the CX231 is attached to the bottom of a Canon 5D MarkII (note, you can still access the 5D battery compartment).
Choice of which juicedLink preamp to use depends on what preamp features you would like (2 vs. 4 XLR inputs, phantom power).
The DN101 can be used with any DSLR camera (7D, GH1, K1, etc ...).
It screws into an existing threaded nut in the bottom of the juicedLink low-noise preamps. The DN101 utilizes the battery in the preamp (small modification to the battery tray required).
The DN101 adds a meter, headphone monitor, and a way to "disable" the camera's AGC to provide significantly better signal-to-noise performance.
The output of the preamp plugs into the input of the DN101. The output of the DN101 plugs into the mic input of the camera. The DN101 also has a headphone jack.
Improving SNR Performance - Audio Clip
The DN101 sends a signal to the camera, so the camera perceives that there is a large signal, and the AGC circuitry throttles back its amplifiers. This has become known as "AGC Disable" for these DSLR cams. The camera amplifiers are very noisy, so this is good. But, it also turns down the level of the microphone signal, which is bad. But, with the juicedLink low-noise preamp at the front end, just simply turn up the clean gain of the juicedLink low-noise preamp to get the mic signal well above the noise in the camera, and the result is significant improvement in system signal-to-noise performance.
Listen to a MP3 audio clip using a juicedLink CX231 with a juicedLink DN101 along with a Canon 5D MarkII (running the standard Canon firmware):
Alternative content
DN101 does NOT inject a tone into your camera
Injecting a tone into an audio system is dangerous! A tone is a coherent signal and easily picked up by the human ear, even at very low levels.
Just because you can't hear the tone, doesn't mean others won't.
YOU ARE DEPENDING ON NON-SPECIFIED ASPECTS OF THE CAMERA: You really need to understand digital signal processing to take the risks of using a tone. You can take a setup that works OK on one camera, and put it on another camera, and it doesn't. Typically, people will try and inject a tone around 20KHz, where most people won't hear it.
1) Cameras have an analog-to-digital converter (ADC). Unfiltered, ADC's will take signals that are out-of-band (where you can't hear them), and fold them back IN-BAND. This is known as aliasing. A well designed camera will have a good Nyquist anti-aliasing filter before the ADC. So, if you are injecting a tone, you are depending on the effectiveness of the Nyquist anti-aliasing filter. If the filter isn't working well, then a harmonic of that 20KHz tone will get folded back IN-BAND where it can be heard.
2) Also, the roll-off of these Nyquist anti-aliasing filters will typically be around 16KHz. So, you're injecting a tone on the steep slope of the filter. A small change in the roll-off frequency will result in a significant change in amplitude of your tone.
3) Additionally, crosstalk in your camera can make your tone, harmonics, or aliased artifacts jump recording channels between the camera's left and right channel.
4) ALL of these things are going on, while you have no way to monitor with headphones IN THE CAMERA to hear what it is doing.
5) Even after you're done, and you listen to your recording in your editing system, and you think it sounds good. You could upload your video to a service like youtube/vimeo/etc, and they could compress your file using an encoding scheme that aliases your tone IN-BAND where everybody can hear it.
Switch it on, and it works
The juicedLink approach for AGC Disable does not use a tone, so you won't fall victim to these issues.
When you switch on the AGC Disable, the DN101 automatically re-calibrates the meter for the appropriate signal levels (you will be cranking up the clean gain in the juicedLink preamp when AGC Disable is on, which throttles back the noisy amps in the camera).
The DN101 AGC Disable will take up one of the recording channels in the camera (right). When the AGC Disable is activated, the DN101 headphone jack will only monitor the left input channel (through both ear-cups of the headphone). So, you will not hear anything in the headphones that is sent to the DN101 right input (as this channel will be used for the AGC defeat). This keys you to make sure that you have all of your mics panned left (the clean recording channel in the camera). For example, say you are using a CX431 with 4 mics. You will set the CX431 pan switches to 'left', and all 4 mics will be recorded by the camera on that track.
The Tradeoff
The design objective for the DN101 was to make this as small as possible, so it doesn't add much to the juicedLink preamps. From the photo, you can see that we have successfully achieved this objective. But, to do this, we needed to make some compromises.
The headphone amp is not what you would consider a "low-noise" amp. Its fine for field work to make sure your subjects are miked up correctly. Obviously, the noise in the headphone amp does not get recorded by the camera. Also, the headphone is a mono monitor.
DN101 Tour
Robert Rozak, the creator of the JuicedLink product line explains how the DN101 works with DSLRs and the JuicedLink preamps to input high quality sound. He demonstrates installing the DN101, proper switch position, the method of controlling Auto Gain Control and the means of monitoring it all with headphones.
Evil Tones
JuicedLink manufacturer, Robert Rozak explains why other methods of using tone to override AGC on DSLRs is inferior to his method via the JuicedLink DN101.
Juicedlink DN-101 Manual (777K PDF)
