THX features synopsis
Home THX Controller
- electronic crossover
- 80Hz subwoofer -24dB/octave LPF
- 80Hz front LCR -12dB/octave HPF
- 125Hz surrounds -12dB/octave HPF
- X-curve Re-EQ on front LCR channels (close to a -3dB/octave LPF at 5kHz)
- Timbre Matching Circuit for surrounds
- THX Decorrelation Circuit for surrounds
- No mention of AC3 Bass Management or the Bass Peak Limiter?
Home THX speakers
- "All the speakers must produce high sound pressure levels ( 105 dB),
without signs of physical or response deterioration."
- Horizontal radiation pattern results in a "reduction of the ceiling
reflection in a Home THX LCR speaker. The size of the ceiling reflection is
almost 10 dB less than the wide directivity speaker." I have a hard time
- Dipole surrounds 125Hz - 8kHz frequency response which is the encoded
surround bandwidth for Dolby Prologic. I think this has been updated to
80Hz - 20kHz for 5.1 discrete source with their full bandwidth surrounds.
- subs flat in room to 20Hz and be able to play 105dB, typically two 12"
drivers are required.
- This is a secret but the THX spec for subs is believed to say a Qtc=.707
optimum alignment and an F3 (-3dB point) at 35Hz.
- IMO with the existence of the +10dB LFE channel the THX subwoofer spec
should really be ammended to require 115dB levels. This will never happen
because this would require from 4 to 8 THX approved subs (this is equal to
8 average 12" drivers). It is out of control, just try selling this to
General Home THX reqs / suggestions
- ideal room 16 x 24 x 8 = 3000 cubic feet
- 30 degree TV viewing angle
- "100W amp be able to drive each speaker to 100dB continuous 105dB peak
in th e far field (2m?) of a 3000 cf room."
- An auditorium cannot exceed NC-30 in any octave band.
- THX mixing facilities must not exceed NC-25.
- Spec is vague, no RT60 times are mentioned.
- It only says "must be within required limits calculated for each
- Viewing Angle
- "The recommended audience viewing angle for the Cinemascope image
(2.35:1) from the screen to the farthest seat in the auditorium is 36 degrees
with 26 degress as the aceptable minimum."
- "Projectors must be placed on center with the image (± 3% optimal with a
maximum of ± 5% allowed). All SMPTE projection standards must be met. In
addition, screen illumination must also conform to SMPTE standards of 16 foot
lamberts (± 4 foot lamberts)."
Problems with Home THX technology
- Not enough bass power with the +10dB LFE channel more subs are
needed. I recommend to increase the number of subwoofers required for THX
certification by rougly 4X the current number for any particular THX approved
- Subwoofer F3 at 35Hz. Having the subs roll off at 30-35Hz might
produce a flat frequency spectrum to 20Hz in the average room, but all rooms
are not the same. I don't agree with this philosophy. I feel a lower F3
(-3dB point) is called for and any excess bass be equalized flat.
- Not enough boost/cut gain control in THX equalizers. Choosing a
plus/minus 6dB gain control range is adequate for most situations and it
prevents the untrained operator from getting into too much trouble. But a
range of 6dB boost or cut is not enough to tame the first axial modes
in some rooms. I've seen humps of 10dB or more. I suggest a plus minus 12dB
range in the bass range, I could be persuaded to agree that -12dB cuts and
only +6dB of boost is an acceptable alternative.
- EQ frequency bands not wide enough. The THX crossover F3 points
are at 80Hz usually. Having the bass EQ band go from 20-80Hz and the front
speaker bands go from 80-800Hz is just not wide enough. I feel that the EQ
bands should extend one octave beyond the F3 cut off points. A strong room
mode at 100Hz will color the subwoofer becuase the -24dB/octave crossover
slope will only be -8dB down at 100Hz, now the room mode hump is +10dB or
more, you have efectively given the sub a 100Hz crossover point. This effect
makes blending of the sub into the front speakers troublesome. This effect
is very audible in some rooms and it makes the subwoofer more localizable.
Again my recommendation is to extend the EQ frequency bands one octave past
the F3 crossover point. 20-160Hz for the subs and 40-800Hz for the front
speakers. Actually 10-160Hz range for the subs would be even better, although
this would be impractical due to low frequency reproduction efficiency and
that most of the necessary tools for calibration do not go that low.
- Limited surround speaker bandwidth. With AC3's discrete channels,
surround speakers should be full bandwidth like the front LCR speakers,
80Hz - 20kHz.
- Surround speaker timbre matching is a compromise. It is designed
for the average room. In my opinion the surrounds should require equalizers
Check out my home theatre tips page.