Lynn Olson
Active Member
- Joined
- Aug 29, 2010
- Messages
- 98
The patent isn't 100% clear on the nonlinear function between the control logic (3-axis direction sensing) and the presumably linear control voltage of the VCA's. Zero volts on the three control lines represent no dominant direction sensed, so the matrix coefficients are the same as a static SQ (or QS) matrix. Each control line has a plus or minus direction, depending on the magnitude and sign of the CF/CB, LF/RF, or LB/RB axis detected on the Scheiber sphere. (This is similar to the phase of the chroma subcarrier in NTSC or PAL color television representing hue, and the percentage magnitude of the subcarrier representing color saturation. Analog color TV is a 2-axis system, while SQ is a 3-axis system, which is why Scheiber notation is needed to understand it.)
But this leaves the exact relationship of the control magnitude and the percentage gain of the VCA's a nonlinear relation. Well, in practice, a string of resistor/diode pairs creates the quasi-exponential part of the curve ... but was the exact shape determined?
Fortunately, this was pretty easy, although not described in the patent (I guess it should have been, but it wasn't). All it takes is a scope with X/Y inputs and a pair of oscillators that are reasonably stable. Set both oscillators to 1 kHz at a moderate level (do not clip the sensing logic or the overall decoder), and offset them by about 5~10 Hz, which is well within the response speed of the direction-sensing logic. As you can imagine from the above drawings, this creates a rapid circular pan from CF -> RF -> CB -> LF -> CF and so on.
1) Connect the LF/RF control line to the horizontal axis of the scope, and connect the vertical amplifier to one of the decoder outputs, starting with LF output (all four will be examined in the same way). The 5~10 Hz offset will swing the display from left to right, and the decoded output will drive the vertical. What you will see is a smooth ramp as the output drops from LF to RF (no dynamic decoding in this region), and a suppressed region as the output swings towards CB (it should be zero at CB, and rise again as it approaches LF). Connect the vertical amplifier of the scope to the RF output; you should see the same thing, just on the other side of the display. Both LF and RF suppression of CB should be mirror-images of each other.
2) Connect the vertical amplifier to LB; the pattern should now be different, with the entire front half of the Scheiber sphere suppressed, and the magnitude only increasing as the incoming signal swings toward CB. This lets you examine the smoothness of the suppression region, and the accuracy of the nonlinear control function (the resistor/diode string). Our prototype, with a 6-pole passive phase shifter, had between 35 to 45 dB of suppression across the entire frontal arc. Connect the vertical amplifier to the RB output; you should see the same pattern ... in fact, this is a good opportunity to make sure that both LB and RB have exactly equivalent patterns. We used a Tektronix dual-beam scope to look at both LB and RB simultaneously, so we could exactly match the two nonlinear functions to each other, and make sure the entire system had symmetric suppression patterns with no bumps or nonlinearities present.
3) The rest is easy. Just use the same transfer function for the remaining 2 axis, CF/CB and LB/RB. Confirm by connecting the CF/CB line to the horizontal input of the scope, and looking at the four output channels again. Look for symmetry and any bumps in the suppression region. When everything is symmetric and suppression is uniform, you're done, except for minor magnitude correction in the decoded channels to keep total magnitudes of the whole decoder exactly matching a static decoder (we didn't find the less-than-1dB correction to be audible).
This should illuminate what I mean by "symmetric" ... it's actually a calibration function of the real-world decoder, and it dynamically measures separation across an entire arc of localizations. I'm not sure anyone else every did anything like this; they were all focused on corner separation, with a nod to preserving overall energy through the decoder. As you can see, if the action of the dynamic decoders is accurate, it not only preserves energy levels in that channel (following an ideal cardioid pattern) but preserves overall energy as well.
This is why it sounds more spacious than other decoders, including, I suspect, modern DPL-II (music) and DTS Neo:6. I don't hear equal energy reverberation through these, and based on modern loudspeaker layouts for home theatre, I don't think symmetric distribution of energy is a design goal.
Theatre systems are, by design, asymmetric, and are asymmetric in the home, as well. This is radically different than the design goals of the better quadraphonic systems of the Seventies, which were designed exclusively for home use. They didn't get re-purposed for theatre use until the late Seventies, when the QS system was re-aligned for a L/C/R/surround format, and after that, Dolby Digital, DTS, and the Sony system.
The requirements for a theatre system are different because nearly all the audience is listening off-center ... the "sweet spot" we use at home is less than 1~2% of the seats in a theatre. The result is no phantom imaging; everything jumps into the nearest speaker. This is why a movie theatre MUST use a center speaker for spoken (or sung) dialog; it sounds really weird if the critical dialog seems to come from the L or R speaker for the whole movie; it would be very, very distracting. This is why all theatres, going back to the dawn of sound movies, have 1 speaker, 3 speakers, or 5 speakers directly behind the perforated screen. It's the only system that isn't distracting.
As a result, the special effect of a moving pan requires lots of speakers, overhead as well as the sides and back. It's the only system that works when nearly all the audience is sitting off-center.
Quadraphonic, by contrast, is a refinement of domestic stereophonic sound, and smooth, evenly distributed phantom images are essential to the whole system. A theatre already sounds large ... because it is ... while a domestic system has to reproduce the spatial impression of the recording, which can be any size, and often completely artificial (EMT plates, digital reverb, etc.).
But this leaves the exact relationship of the control magnitude and the percentage gain of the VCA's a nonlinear relation. Well, in practice, a string of resistor/diode pairs creates the quasi-exponential part of the curve ... but was the exact shape determined?
Fortunately, this was pretty easy, although not described in the patent (I guess it should have been, but it wasn't). All it takes is a scope with X/Y inputs and a pair of oscillators that are reasonably stable. Set both oscillators to 1 kHz at a moderate level (do not clip the sensing logic or the overall decoder), and offset them by about 5~10 Hz, which is well within the response speed of the direction-sensing logic. As you can imagine from the above drawings, this creates a rapid circular pan from CF -> RF -> CB -> LF -> CF and so on.
1) Connect the LF/RF control line to the horizontal axis of the scope, and connect the vertical amplifier to one of the decoder outputs, starting with LF output (all four will be examined in the same way). The 5~10 Hz offset will swing the display from left to right, and the decoded output will drive the vertical. What you will see is a smooth ramp as the output drops from LF to RF (no dynamic decoding in this region), and a suppressed region as the output swings towards CB (it should be zero at CB, and rise again as it approaches LF). Connect the vertical amplifier of the scope to the RF output; you should see the same thing, just on the other side of the display. Both LF and RF suppression of CB should be mirror-images of each other.
2) Connect the vertical amplifier to LB; the pattern should now be different, with the entire front half of the Scheiber sphere suppressed, and the magnitude only increasing as the incoming signal swings toward CB. This lets you examine the smoothness of the suppression region, and the accuracy of the nonlinear control function (the resistor/diode string). Our prototype, with a 6-pole passive phase shifter, had between 35 to 45 dB of suppression across the entire frontal arc. Connect the vertical amplifier to the RB output; you should see the same pattern ... in fact, this is a good opportunity to make sure that both LB and RB have exactly equivalent patterns. We used a Tektronix dual-beam scope to look at both LB and RB simultaneously, so we could exactly match the two nonlinear functions to each other, and make sure the entire system had symmetric suppression patterns with no bumps or nonlinearities present.
3) The rest is easy. Just use the same transfer function for the remaining 2 axis, CF/CB and LB/RB. Confirm by connecting the CF/CB line to the horizontal input of the scope, and looking at the four output channels again. Look for symmetry and any bumps in the suppression region. When everything is symmetric and suppression is uniform, you're done, except for minor magnitude correction in the decoded channels to keep total magnitudes of the whole decoder exactly matching a static decoder (we didn't find the less-than-1dB correction to be audible).
This should illuminate what I mean by "symmetric" ... it's actually a calibration function of the real-world decoder, and it dynamically measures separation across an entire arc of localizations. I'm not sure anyone else every did anything like this; they were all focused on corner separation, with a nod to preserving overall energy through the decoder. As you can see, if the action of the dynamic decoders is accurate, it not only preserves energy levels in that channel (following an ideal cardioid pattern) but preserves overall energy as well.
This is why it sounds more spacious than other decoders, including, I suspect, modern DPL-II (music) and DTS Neo:6. I don't hear equal energy reverberation through these, and based on modern loudspeaker layouts for home theatre, I don't think symmetric distribution of energy is a design goal.
Theatre systems are, by design, asymmetric, and are asymmetric in the home, as well. This is radically different than the design goals of the better quadraphonic systems of the Seventies, which were designed exclusively for home use. They didn't get re-purposed for theatre use until the late Seventies, when the QS system was re-aligned for a L/C/R/surround format, and after that, Dolby Digital, DTS, and the Sony system.
The requirements for a theatre system are different because nearly all the audience is listening off-center ... the "sweet spot" we use at home is less than 1~2% of the seats in a theatre. The result is no phantom imaging; everything jumps into the nearest speaker. This is why a movie theatre MUST use a center speaker for spoken (or sung) dialog; it sounds really weird if the critical dialog seems to come from the L or R speaker for the whole movie; it would be very, very distracting. This is why all theatres, going back to the dawn of sound movies, have 1 speaker, 3 speakers, or 5 speakers directly behind the perforated screen. It's the only system that isn't distracting.
As a result, the special effect of a moving pan requires lots of speakers, overhead as well as the sides and back. It's the only system that works when nearly all the audience is sitting off-center.
Quadraphonic, by contrast, is a refinement of domestic stereophonic sound, and smooth, evenly distributed phantom images are essential to the whole system. A theatre already sounds large ... because it is ... while a domestic system has to reproduce the spatial impression of the recording, which can be any size, and often completely artificial (EMT plates, digital reverb, etc.).