Dr. AIX's POSTS — 04 March 2015

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I had a series of back and forth emails with a fellow audiophile yesterday regarding the LFE levels on my calibration and alignment disc. He wrote to me and asked if I had reduced the level by 10 dB as recommended and required for home theater systems. I double-checked the tones of the “AIX Records Calibration and Demonstration Disc” by playing the test tones in my studio and reading the levels. It turns out that I didn’t attenuate the LFE channel levels. And he believes that I’m violating the technical requirements of a proper home theater setup. I disagreed and tried to explain why…he was not convinced. I spent so much time trying to answer his questions and explain the use of the LFE and subwoofer that I thought I would discuss the topic today.

Before we get into the nitty gritty this topic, let me first start with the concept of stereo panning. Trust me it’s relevant. There is a rotary knob on recording consoles called a panner. Whenever the “stereo bus” or main mix outputs are engaged, the panner allows the engineer to distribute the audio signal coming through a particular input channel anywhere in the Left/Right stereo field. Imagine the track is a cowbell. During the mixing process the engineer can pan the bell to the extreme left or right side. These special cases would place all of the amplitude of the cowbell in a single speaker.

However, if the mixer decides that the cow bell belongs in the center of the stereo mix, he or she will use the stereo panner to move the cow bell so that it appears to come from the between the left and right speakers. This is called “phantom center”. We hear the sound coming from the center but in fact, it’s coming from the left and right speakers with equal signals level…but not the same signal level that it had when it was panned to the extreme left or right side. Why has the amplitude changed? Has it been lowered or raised?

It’s been lowered by 3 dB when the image is panned to the center because now there are TWO sources for the sound. And when a sound is output from two speakers, the level of the two signals combines and increases by 3 dB. So the panning circuits built into the channel inputs of the console strips has a smooth attenuation of 3 dB when panned to the center position. Look at diagram below:

150304_panning_diagram

Figure 1 – An illustration showing how a panner attenuates a signal when in the center.

Engineers don’t have to worry about the center “build up” because the electrical engineers designed the circuits to avoid the problem.

More to come…

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About Author

Dr. AIX

Mark Waldrep, aka Dr. AIX, has been producing and engineering music for over 40 years. He learned electronics as a teenager from his HAM radio father while learning to play the guitar. Mark received the first doctorate in music composition from UCLA in 1986 for a "binaural" electronic music composition. Other advanced degrees include an MS in computer science, an MFA/MA in music, BM in music and a BA in art. As an engineer and producer, Mark has worked on projects for the Rolling Stones, 311, Tool, KISS, Blink 182, Blues Traveler, Britney Spears, the San Francisco Symphony, The Dover Quartet, Willie Nelson, Paul Williams, The Allman Brothers, Bad Company and many more. Dr. Waldrep has been an innovator when it comes to multimedia and music. He created the first enhanced CDs in the 90s, the first DVD-Videos released in the U.S., the first web-connected DVD, the first DVD-Audio title, the first music Blu-ray disc and the first 3D Music Album. Additionally, he launched the first High Definition Music Download site in 2007 called iTrax.com. A frequency speaker at audio events, author of numerous articles, Dr. Waldrep is currently writing a book on the production and reproduction of high-end music called, "High-End Audio: A Practical Guide to Production and Playback". The book should be completed in the fall of 2013.

(18) Readers Comments

  1. Mark this is good stuff. Especially for a guy who is wondering how to setup a home sound room. I say sound room because I am not yet sold on the benifts of 5 speakers.

    • Scot, before you setup your room you should visit a friend or a high-end retailer and check out a proper 5.1 surround music system. Take some files or a disc of music that you like in 5.1 surround, listen and then decide.

  2. Yes, there is a lot of misunderstanding and mis-information on the subject of subwoofer set-up. I have been to “THX School.” I have set up dozens of HT systems. The test tones, etc., won’t really help, and I wouldn’t “delay” your sub 1 ms, it’s already the ‘slowest’ speaker you own.

    Put on familiar music that has powerful, extended bass lines and LF events which have true”start/stop” transient qualities. Set your speakers and sub up to deliver A-grade performance on musical bass transients that give pitch, attack, and definition to these usually muddled notes. Remember, organ pedals stop and start too, and precision low frequencies make this obvious and startling, as occurs in real life.

    If your system is properly balanced at that point, get out your movie material and see what you hear on LFE passages; if you like a little extra “hot fudge” on your HT sundae, go ahead, but do not lose the qualities needed for precise reproduction of musical bass transients.

    • Subwoofer and the amount of low frequency energy is very tough to get right…especially since the amplifier is in the SUB and not in the AVR. We do align our subwoofer output using a sound pressure meter because our mixes have to translate to other rooms (specifically move theaters) and they count on correct levels.

  3. Actually, Mark, your correspondent is correct about the correct level of the LFE signal. The LFE is not the same as bass management. The sub handles both jobs, but they are conceptually distinct and can be set independently.

    LFE stands for “Low Frequency Effects” and was introduced by the movie industry not for faithful reproduction of music but for the loud bangs and booms in movies. It’s the “.1” track. The LFE track is deliberately mixed 10db low to allow headroom for the loudest sound effects. It has to be boosted by 10db on playback for precisely that reason.

    The sub does double duty, however. It not only carries the LFE but also the sounds below 80hz from the surrounds and often the front three speakers as well. The standard crossover point for bass management in movies is 80Hz, a standard promulgated by George Lucas as part of the THX calibration of movie theaters. Anyone setting up a home theater will most likely use that crossover point for compatibility with movies.

    The rule followed by AV Receivers is to boost the decoded LFE signal from the soundtrack by 10db. There’s no similar rule for the redirected bass, which is more often adjusted “to taste.” As a mixing engineer, your insights into panned signals is applicable to the redirected (or “steered”) bass, but the LFE 10db cut and boost is a different matter.

    This same schema is followed by Dolby and DTS. The exception is SACD, which, not being a movie system, does not boost its .1 signal, since it’s not really an LFE signal but part of the music, and thus doesn’t get the reciprocal 10db cut and boost. (And in fact, many multichannel classical recordings are in “5.0” – no subwoofer signal other than your own bass management.)

    Anyone who uses the multichannel analog output of a disk player to connect to an AVR has to make these adjustments manually in the AVR, since it’s not doing the decoding and can’t know what the encoding was. It has to be done in the AVR because if you boost the .1 output inside the player, you’ll likely overload either its output stage or the AVR’s input.

    And if you’re running that way, setting the LFE level manually, SACDs will sound boomy until you remember to cancel the 10db boost to the sub!

    • Thanks Phil…based on several comments and emails, I’m still looking in the topic. Your explanation is quite good. I think my wording of the LFE channel was part of my mistake. I don’t mix LFE for movies, my mixes have a “boom” channel output from my console that is not bass managed and is not attenuated.

      • Mark, since the 10db boost applied by AVRs that do the decoding themselves is hard-wired and can’t easily be changed by the consumer without turning down the total signal to the subwoofer, drastically cutting the volume of the steered bass, you may not *want* to apply attenuation to your .1 channel in making your disks but you *have* to.

        It doesn’t matter if you use Dolby TrueHD, DTS HD Master Audio, or LPCM – the AVR will always boost the .1 channel by 10db unless there’s a configuration setting on sub obscure menu to undo that 10db boost.

        So if you don’t attenuate your .1 channel it will in most cases come out too loud because the AVR will boost it because it’s a non-SACD “.1” channel and that’s what the specification directs for all codecs other than SACD.

        Why not try A/Bing the original mix of one of your Blu-rays that has significant activity on the .1 track and the playback of that disk by one of your Oppos. If the playback sub sounds the same in relation to the rest of the speakers, I’m wrong. If it sounds a lot louder, I’m right.

        You’re one of the few folks who could actually easily run such a test!

        • It just occurred to me that DVD-Audio, which (like SACD) was designed for music, not movies, may not apply the reciprocal 10db cut-and-boost to the .1 channel.

          On the other hand, from what I’ve heard, TrueHD is based on DVD-A. Whether it had to add the cut-and-boost or whether DVD-A already used it I don’t know.

          But when you use Blu-ray as your distribution medium, it comes with the territory.

        • I’m digging in to this..and learning. I can do the test you’re suggesting. Stay tuned. From what I hear right now, I don’t lower the sub channel and the mixes sound correct.

  4. The LFE channel might be a good idea for movie sound effects, but it doesn’t let music sound natural, at least not with my sound system. I prefer to connect my subwoofers (yes plural) to the front speakers and turn off the LFE channel. Music sounds better this way. I adjust all the surround speaker levels using pink noise and a real-time analyzer with A weighting with the subwoofers off, and I adjust the gain of the subwoofers using pink noise and a real-time analyzer with C weighting. Some early DVDs came with surround alignment programs and I don’t remember them as being that useful. Is there a better way?

    • Dick, I think you’re losing the information in my .1 channel, which has meaningful amounts so very low bass. My own calibration disc has uniform levels on all outputs because my discs don’t get the usual lowering and raising of the .1 channel.

  5. Hi Mark,
    Thank you for addressing this subject. As most of my listening is in 5.1, it is a constant source of annoyance that different formats have different output levels for the LFE channel. SACD is always lower than DVD-A and DTS is different again. It is hardly surprising that surround has not taken off, when we have “format wars”, stereo audiophiles claiming (falsely) that it is a gimmick and the lady of the house not only objecting to five speakers, but wondering why you can’t have the two you already own put behind the curtains?

    • Handling low end signals is a constant struggle. But even without a subwoofer speaker, surround trumps stereo.

    • Put your foot down and let her know the listening room is your man cave and your the designer there. Ask her to go to the kitchen and make you something to eat while you set up the 5.1 system.
      As B B King sings, “As long as I payin the rent and the bills, I payin the cost to be the boss!

  6. Hi Mark,

    If I’m understanding your correctly, then, your are lowering the LFE material by 10db on your discs. So wouldn’t the material contained in the ‘.1’ LFE channel on your discs be 10db too hot if played through a processor that is bumping the level by 10db?

    • I don’t change the level of my bass output channel…it’s the “boom” channel on my console. I’ll be going through the exact procedure…including the encoding method in future posts.

  7. I can hear my subwoofer only by cranking it up to near-maximum — otherwise, my five to seven full-range speakers pump out all the bass available on any given disc or download. It’s so pointless and redundant that I’ve had it unplugged for well over a year — its only purpose now is to act as a table for my tiny AVR control monitor.

    Modern AVRs can do a low-pass filter and send the bass to the subwoofer for folks with puny satellite speakers, so it makes more sense when labels don’t even bother with the subwoofer. And, in fact, most labels go with 5.0, not 5.1 or 7.1.

    • Not sure you’re right here…of the surround music discs that I’ve surveyed, it’s the exception that doesn’t use the subwoofer.

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