Are You So Focused On Video That You’re Neglecting Audio?

How Much Dynamic Range Do You Need?

There are rules of thumb on how much dynamic range you might need for a given event, but it does vary based on what kind of signal it is you're trying to capture. Over time and experience, you'll get to know what levels you need.

There are two basic levels that audio signals get broken into, as shown in the chart in Figure 6 (below). There's mic level, meaning from a microphone, and those are very, very low levels, typically around -60 dBu or -50 dBu. Then there are line levels, and line levels are much higher. They're the things that would come out of your computer output, a mixer, or a tape deck.

Figure 6. Common mic and line levels. Click the image to see it at full size.

Unfortunately, there are multiple standards, or very typical levels for line levels. If someone tells you they're giving you a line-level signal, that's really not enough to know everything you need to know. You have to ask, "What level is that really at?" One of the most popular levels, such as you might get from a pro mixer, would be 4 dBu. They're also very popular at 0 dBu. Those are different values, but they both come from pro-level mixers, and it just depends which make or model you’re using. With consumer-level gear, such as the output from your laptop, a tape deck, or an iPod, the level would be closer to -10 dB.

Figure 6 gives you a quick reference for converting between volts, dBV, and dBU. You can find lots of these converters, either on the internet or even apps for your phone. If you find yourself needing to translate between one spec that says it can only be a maximum of 6.3 volts, and another spec is telling you it's coming out at -8 dBu, and you need to understand whether that's compatible, you need to convert them.

Working With Audio Mixers

Any mixer that you'll run across will have have output specifications. Figure 7 (below) shows an Allen & Heath mixer. The output specification is typically 0 dBu nominal and this one is 21 dBu maximum.

Figure 7. Output specs on an Allen & Heath mixer. Click the image to see it at full size.

The meter in Figure 8 (below) tells me when the sliders are adjusted so that it's not amplifying the signal. It’s just mixing, which is where you normally want to be, and the meters are just in the green stage, and just touching into the yellow. That's normally how you want to run your mixer. That's considered the nominal level, then the output from that, the average value, will be 0 dBU.

Figure 8. An analog VU meter showing a signal where it should be. Click the image to see it at full size.

These analog meters are set up so that the point you want to measure against is called "0 dBu," and that's your nominal level. It's green all the way up to zero, then it goes yellow, then it goes red. In an analog scheme, you're trying to line up your good value to the zero point. In a digital meter, it runs a little bit differently. In the meters shown in Figure 9 (below), the 0 point is at the top, and that is the clipping point. On the digital side, you're trying to stay away from 0.

Figure 9. Analog and digital alignment levels. Click the image to see it at full size.

On the analog meter, you're trying to line everything up to 0 dB. On the digital side, 0 dB is where you're going to clip, and you want to be -18, -15, or -20 DB below that. You typically line up your meters where the analog is running at zero, and the digital is running at about -18. The green LEDs and the yellow LEDs correspond, so at least when you're using the two systems, if you're in the green, you're good on both sides. You just need to be aware that zero is not always good in the digital world.

Let’s consider a typical example from a live event. I’m recording an acoustic guitar and a singer, and the signal is coming in at +4 dBu from the mixer. I have an encoding spec, that says I can't go more than .7 volts RMS, because that's where I'm going to clip, and I have encoder gain that I can use to adjust from 0 to +30 dB. The +4 dBu is way too much for that .707, so I convert the .707 to dBu and that comes out to -1, which means I need a signal that's -1 dBu maximum.

I'm going to get a signal out of the mixer that's already +4 dB at the nominal level, but peaks are going to be above that. I need to insert an attenuator, so I put a put a +25 dB attenuator in there, which pulls the signal way down to -21 dB. This means I can get it into my digital recorder, and then I use the gain setting inside of the recorder to bring that up to -16 dB, and that gives me 15 dB of headroom, which is comfortable for recording that kind of performance.

Basically, what you're trying to do is make sure you understand the specs of the equipment that you're using, properly understand the analog meter and how that works, and how the digital meter works on the other side, and avoid that clipping point. Use compression if you have to, but most times you can just be really conservative on your headroom. Leave lots of headroom and you won't clip.

For more information on this topic, check out the white paper at www.epiphan.com/Audio 101. It goes into compression, limiting, dynamic range, and more in lot more detail.