Wednesday, June 27, 2012

AT835ST AT815ST Are Now BP4029 and BP4027



Audio Technica BP4027
This review was written about the AT835ST and AT815ST. The models were changed slightly in 2007 before being renamed the BP4029 and BP4027, respectively. In the rapidly changing world of pro audio, taking things for granted is not a good idea. If you think you know what to expect from a stereo shotgun mic, you might accidentally dismiss Audio Technica’s 9-inch AT835ST ($899) and 15-inch AT815ST ($999) mics. 

The most obvious thing about these two mics is that, in addition to being regular shotgun mics, they offer both M/S (Mid/Side) and L/R stereo, (with both narrow and wide L/R spreads). You may already have heard these mics. They were used extensively for EFP and stereo ambience during the Sydney Olympics.
Both mics come in a vinyl box with mic clip, foam windscreen and a five pin XLR that plugs into the mic and splits to a pair of three pin XLRs. The diameter of both mics is slightly larger than a Sennheiser 416, but small enough to fit properly in a standard rubber, Rycote boom mount.
Audio Technica BP4029
Both mics use the same two electret capsules; a front facing line-cardioid mid capsule, and a figure of eight side capsule mounted directly behind the mid capsule. Both mics require 11-52 VDC at 4mA phantom power. The longer AT815ST weighs less than five ounces; the AT835ST less than four ounces. This makes them well targeted for the EFP/ENG video markets with their rapidly growing need for stereo ambient sound or any other stereo sound gathering application where weight is a factor.
Both mics have the word "UP" embossed on the body to indicate the correct orientation. Both also have two switches, one engaging a somewhat unconventional (keep reading) 80Hz 12dB/octave LF rolloff filter and one for M/S, LR-W Audio Technica BP4029 (wide) and LR-N (narrow). An internal matrixing circuit converts the M/S signal to the two different L/R spreads. Using the line-cardioid capsule only in M/S mode, I found the sensitivity of both AT mics was one or two dB below my Sennheiser 416 and 816 and the AT mics heard a bit more off the sides and to the rear. 
The level of selfnoise was about the same, although the spectra of the selfnoise was different. The Audio Technica mics made sort of a “hiiiiiih“ compared to the Sennheisers’ “pfffffff.”
ROLLOFF REVELATION
One very noticeable difference between these AT mics and others I have used is in the design of the LF rolloff filters. The flat frequency response of the shorter AT835ST begins a LF rolloff at about 500HZ and gently slopes down -3dB at 70Hz and remains there down to 30Hz. Engaging the LF rolloff switch actually increases the LF response between 100Hz and 500Hz. Below 100Hz it then drops off more steeply at 12dB/octave. This means you get more mid bass and less low bass with the LF filter engaged. 
The longer AT815ST flat response has a 3dB bump in the 30Hz-50Hz range that returns to 0dB at 100Hz. Engaging the LF filter causes a 4dB increase between 100Hz and 400Hz, which then drops off gently at about 10dB/octave below 100Hz. Again, engaging the LF filter causes an increase in upper bass and a decrease in low bass.
Both mics also have presence peaks slightly higher in frequency than that of the Sennheiser 416. The shorter AT835ST begins a slow 2dB rise from 1kHz to about 2.5kHz. It then achieves a +4dB plateau from 4kHz to 7kHz, peaks at +5dB between 8kHz and 9kHz and slopes off moderately crossing zero at 15kHz before dropping down to -4dB at 20kHz. The longer AT815ST begins a gentle rise at 1kHz, hits +4dB at 4kHz, dips a dB or two between 6kHz and 7kHz, rises to a +5dB from 8kHz to 10kHz, is back down to +3dB at 15kHz and slopes off to -1dB at 20kHz.
In their flat positions, both mics sound thin as compared to a 416 Sennheiser. With the LF filters engaged, the increase in upper bass makes them sound more similar. The 416 still has more beef in the upper bass and develops its presence peak a bit below that of the AT mics, making them sound a bit “zippier” on top. In my short time with them, I found that I liked keeping them in the rolled off position to get that extra upper bass, while reducing the amount of low bass.
Audio Technica BP4027 Using the two mics in standard shotgun configuration with only the mid capsule, I found the patterns were slightly wider than the the 416, with typical off-axis shotgun phase anomalies. When operated as M/S or L/R, both mics have a slight preference for sound that’s 30 degrees or so either side of front. Move past that and there’s a slight reduction in HF and clarity. You can get that clarity with something like a Neumann RSM 191, which is, of course, shorter and about four times as expensive. 
I did find that, in stereo operation, as a sound source works its way from front to the rear, there’s a point after the source gets past the side capsule at which the mic gets a bit confused and throws the signal to the opposite side. As the sound source continues past the rear axis of the mic and heads back to the front on the other side, a similar “flip-flop” happens. This happens in either L/R mode or in the M/S I matrixed in my Orban Audicy DAW. So if you’re doing sound work at an automotive road rally, it’s probably not a good idea to set up in the middle of a couple of deep curves that have result in the cars crossing the rear axis of the mic. Y M/S?
If you haven’t considered M/S stereo, it's a handy format when you don’t have a clue as to how wide you want the stereo image in the final mix. Although it’s a bit difficult to listen to in the field unless you can convert it to L/R, being able to adjust the width in postproduction is a definite advantage.
If you're using zone mics, you can use an M/S mic as a stereo center spot mic for soloists, using the matrixed mid channel fader to adjust gain for the performer without upsetting the stereo balance of the side channels. I took the AT835ST to Open Stage night at 8x10, a local music hot spot in Baltimore where Craig Hopwood runs the house and mixes PA. Through the 8x10 sound system, the AT835ST sounded a lot more open and clearer than the Shure Beta 58s. Adding 3-4kHz and10kHz at the board made the Betas sound more similar.
Running the mic in L/R wide, I twisted the mic around 90 degrees so it was aiming high and low instead of L/R stereo. Pointing the mic at about the Adam’s apple of a folk singer, I was able to get a split of more voice on one fader and more guitar on the other. In L/R, harmonized vocals with two singers each about a foot and a half away from the mic also worked well. When placed a foot away from an acoustic 12-string guitar, we got a nice image, but ran into feedback trouble when we added more instruments and had to crank up the stage monitors. Back at the studio I had better results under more controlled conditions. 
IN CONCLUSION
The AT835ST and AT815ST are lightweight mics with obvious EFP/ENG applications, especially with a tailored LF response that eliminates low end. Having a LF rolloff filter that increases the middle low frequencies and dumps the lower ones actually works out to be a nice idea. Having slightly “center focused” stereo images means you’ll get some stereo information, but nothing radical that could cause problems later. With their higher presence peaks, and out of the way of blaring stage monitors, either mic might also find use as a stereo drum overhead mic.

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