Anthony Grimani (email@example.com) is president of Performance Media Industries, with offices in San Francisco, Los Angeles, and Paris, France. Despite our geek-driven desire to impress clients, we often miss the mark. AV gear is really complicated to configure and tune, often unintuitive, and sometimes errant in its menus, claims of performance, or features. I’m going to highlight some things that often go wrong and describe tests to diagnose these problems using basic gear. Let’s start with audio.
1 SOURCE DEVICE SIGNAL CONFIGURATION.
Most source devices come in a default mode where Dolby and DTS bitstreams are converted to 2-channel PCM. Look for an option to change the output format to something called “Bitstream.” Test that you succeeded in changing the format by monitoring the signal status of the decoder. You should also double check using a test disc. The PMI/Gold Line 5.1 Audio Toolkit Title 1, Chapter 9 will tell you audibly if everything is OK. It is midrange pink noise that goes around the room in this order: left, center, right, side right, backs, side left. Each noise burst should come from the correct speaker(s) at about the same level. If the source device is in the wrong output mode, some speakers will be silent.
2 OUTPUT LEVEL ACCURACY.
Surround decoders have built-in test noise for aligning output levels. A good number of decoders, however, produce inaccurate noise levels. Overall levels may be wrong, or some channels may be higher or lower than standard. Check this out by using the same Toolkit Chapter 9 mentioned above along with a Radio Shack or comparable SPL meter. Every channel should be 75dB, C weighted, Slow. You can also check each channel individually by starting at Chapter 1. Go around the room one channel at a time, including the subwoofer. While you are at it, note what master volume level yields the 75dB reference level. Tell your clients to listen at this level for a true theatrical experience.
The tonal balance of any speaker is affected by the room and must be corrected. Unfortunately, automated systems can’t be trusted.3 FREQUENCY RESPONSE ACCURACY AND CONSISTENCY.
The tonal balance of any speaker is affected by the room and must be corrected. Unfortunately, automated systems can’t be trusted. You must verify their results. Chapter 17 cycles wideband pink noise between the front speakers. They should all sound very similar to each other and to high-accuracy earphones like the Etymotic ER4S. You can also measure and compare the frequency response using basic analyzer tools for your smartphone. Most smartphones have bass filters, so you won’t get good results below 100 Hz. Personally, I prefer manual equalization with more sophisticated analysis tools and several hours of technician time. Chapter 19 cycles wideband pink noise around the whole room, so you can check the surround speakers, too.
4 TIME DELAY ACCURACY.
Most surround decoders let you set the distance to each speaker to automatically delay the closer speakers to match the furthest speaker. Sometimes, it doesn’t work right. Chapters 20-26 produce a signal at the midpoint between each speaker pair. You should clearly hear a phantom image right there. Try increasing or decreasing the distance settings to see if you can improve the phantom images. Once they are locked in, the soundfield develops into a gloriously enveloping bubble!
5 BASS MANAGEMENT.
In 99 percent of systems, you should set all the speakers to “small” and feed the bass to a subwoofer system in mono. There is no good reason to use behemoth speakers around the room. It just doesn’t work due to practicality and standing wave interference. But how sure are you that the bass is properly redirected to the subwoofer? If it isn’t, you run the risk of blowing the speakers. Play Chapter 19 and disconnect the main speakers. The subwoofer should play a consistent level and character of bass as the sequence goes around the room. Compare this to Chapter 16, which is the LFE channel only, coded at a level that should match Chapter 19.
6 BASS HEADROOM.
Subwoofers should be able to play 110dB (105dB minimum). Chapter 79 will feed really loud LFE bass to the subwoofer. Use an SPL meter to check the results. If you don’t hit the 110dB mark, you may need a better subwoofer. It may also help to filter out the infrasonic range, check the wiring and AC power, or reposition the subwoofer so it drives the room more efficiently./p>