BASS MANAGEMENT SYSTEM

Transcription

1 E BASS MANAGEMENT SYSTEM 1 What is Bass Management? In stereo reproduction, signals from 20Hz to 20kHz need to be replayed. Multi-way speaker systems will reproduce that bandwidth evenly. With multichannel audio, professional and consumer applications must be able to reproduce all frequencies from each channel. Main speakers, subwoofers and crossovers should work together to give a flat response for each channel. In small studios with basic acoustic treatment and geometry, the frequency response of a speaker system below 100Hz is dominated by the modal response of the room. Strong low frequency standing wave patterns can be observed in small rooms that have parallel walls. Therefore, it is very difficult to achieve consistent low frequency response from multiple full-range speakers in such limited space. One solution to this basic acoustical problem is to employ a system called Bass Management. Using active electronic filters and crossovers, one can extract the low frequency information from the main channels and route that information to a single subwoofer feed. The low frequencies are now originating from one single source that can be placed in an optimum position in the room. Furthermore, the channel can also be monitored via this subwoofer and added to the low frequencies of the other main channels. Therefore, the Bass Management s basic and main goal is to ensure that the entire audio bandwidth of all channels can be accurately monitored. Dolby aboratories Bass Management allows the user to redirect low-frequency information from any of the main speakers to the subwoofer. Tomlinson Holman, TMH aboratories Bass Management in monitoring can be used to reproduce both the very low frequency content of the main channels, as well as the channel, over one or more subwoofers. 2 Bass Management and Small Monitors If the multichannel set-up uses small or mid sized monitors, then a subwoofer should be included to extend the frequency response of all main channels and correctly monitor the channel. The F cut-off of the subwoofer in the room should be close to 20Hz. As mentioned earlier, having one source of omni-directional low frequency information in a small room is usually quite beneficial as this simplifies the F radiation pattern and enables the user to find the optimum position for the subwoofer. The second benefit, as discussed earlier, is to avoid the F cancellation due to reflections off the walls behind the monitors. oncerning the subwoofer itself, the low-pass filter of the subwoofer crossover should be very steep so that midrange information is sufficiently attenuated. Now, for the subwoofer bandwidth, a fixed crossover point has been carefully chosen at 85Hz. This means that for the Bass Management function of the six main channels, the crossover point between the subwoofer and the main channels is set at 85Hz. If we look at the bandwidth of the channel after encoding, (see hapter D1) we observe that this bandwidth can vary quite significantly. For that reason, the Bass Management for the channel bandwidth provides three different setting possibilities for reproduction. The setting of the subwoofer bandwidth will not at all affect the encoding of the channel in the various formats, but does provide different replay bandwidths. Genelec Oy, 2002, Version 3 Page 30

2 3 Genelec 6.1 Bass Management System To provide an integrated solution the Genelec 6.1 Bass Management System is built-in the 7060A, 7070A and 7071A subwoofers. The active electronics of the subwoofer part is combined with the bass management thereby offering a very versatile system. The 6.1 Bass Management system features: Six main channels of inputs & outputs are provided for low frequency redirection. Each channel is identical and has a fixed crossover frequency set at 85Hz. Bass management Bypass Function is provided for the six main channels. The channel is not affected by the bypass function, regardless of the bandwidth and redirection settings. The bypass function can be activated via the remote J11 connector or the jack bypass input. Discrete input with user-selectable reproduction bandwidth: Upper cut-off at 85Hz or at 120Hz Possibility to redirect (on/off) the information above 85Hz to the entre channel when the bandwidth is set to 85Hz. Selectable +10dB gain for the channel. This can be selected on the bass management front plate of the subwoofer or remotely via the J11 connector. Slave subwoofer function, selected with the Sum In DIP-switch. This selection must be done to guarantee proper operation and all other switches of that group should be set to the OFF position. The slave subwoofer receives only one input ( IN/SUM IN) and follows automatically the functions selected in the master subwoofer. The discrete Sum Out output sends the total subwoofer signal to another subwoofer(s). This gives the possibility to daisy chains multiple subwoofers to achieve higher SP at the listening position relative to the room size. The three-colour power ED indicates signal On, lipping and Driver Protection. The ED can also be remote using the J11 connector. DIP-switch settings to adjust the phase at the 85Hz crossover frequency between the subwoofer and the satellite monitors. A built-in 85Hz test tone generator provides a tool for easy on-site adjustment. Tone controls: Bass roll-off adjustment of 2dB, 4dB and 6dB. Variable input sensitivity from 6dBu to +12dBu The typical connection of the Genelec 6.1 Bass Management is shown in Figure 14. All channels including the channel are connected through the Bass Management. The main six channels are then connected from the output of the bass management to each of the main monitors. A single output from the master subwoofer (Sum Out) is fed into the slave subwoofer unit ( In/Sum In). Genelec Oy, 2002, Version 3 Page 31

3 HFO FMFD HFO FMFD HFO FMFD HFO FMFD HFO FMFD HFO FMFD Speaker Set-up for Multichannel Monitoring Slave Subwoofer S S Figure 14 - onnection to the Genelec 6.1 Bass Management. 3.1 Full ange Input The first possibility for the input bandwidth is the reproduction of the channel by the subwoofer up to 85Hz and then the redirection of the energy above 85Hz to the centre channel. This means that all information present on the channel will be reproduced through the monitoring system, whatever the upper cut-off frequency of the channel see Figure 15. Any noise, distortion artefacts and other unwanted sounds on the channel can be monitored in addition to the signal itself. In addition, any type of content (coherent or non-coherent audio material) will be properly monitored without unpredictable summing of the low frequencies between the different channels. This should be the default setting for the subwoofer as it gives full range monitoring of the channel. Genelec Oy, 2002, Version 3 Page 32

4 20Hz 85Hz 20kHz Mixing Desk Multichannel Monitoring Section O Surround Sound Decoder Outputs S S edirection to enter hannel Subwoofer Figure 15 - Multichannel Monitoring with Genelec 6.1 Bass Management. edirection of the channel above 85Hz to the entre channel. 3.2 Input Band-limited to 85Hz The second possibility for the subwoofer bandwidth is the reproduction of the channel by the subwoofer up to 85Hz, without any redirection above 85Hz to the centre channel. This setting is comparable to pre-filtering the channel in the mixing console using an 80Hz P filter. No information above 85Hz present on the channel will be heard through the monitoring system see Figure 16. This setting is not recommend for everyday use as the channel has a bandwidth up to 120Hz on Dolby Digital and DTS, and wider bandwidths some of the other formats. A good use for this setting is to simulate the effect of some (usually cheaper) consumer decoders that do not replay the information on the channel that is above 80Hz when the bass management is used. It is important for the mixing engineer to be aware of such a limitation so that the multichannel mix translates well in the home environment. This configuration provides a replica of what will happen in many home surround system situations and yields to consistent reproduction of the low frequencies below 85Hz. Also, any type of content (coherent or non-coherent audio material) will be properly monitored. As mentioned earlier, Dolby and others recommend that, for DVD mixes for the home, the channel is monitored and recorded onto tape with an 80Hz low pass filter inserted in the console output buss. This setting is a quick and easy way to simulate the monitoring part of that recommendation. Genelec Oy, 2002, Version 3 Page 33

5 20Hz 85Hz 20kHz Mixing Desk Multichannel Monitoring Section O Surround Sound Decoder Outputs S S Subwoofer 85Hz cut-off NO edirection to enter hannel Figure 16 - Multichannel Monitoring with Genelec 6.1 Bass Management. Input band-limited to 85Hz. 3.3 Input Band-limited to 120Hz The third possibility for the subwoofer bandwidth is the reproduction of the channel by the subwoofer up to 120Hz without any redirection above that frequency. This means that no information above 120Hz on the channel will be heard through the monitoring system see Figure 17. This setting has been provided so that the production facility can emulate the replay systems that exist in Movie Theatres and inemas. In this application, there are strict rules on how the replay system should perform. Dedicated subwoofers with a bandwidth from 20Hz 120Hz reproduce the low frequency content for the 35mm movie soundtracks. However, it must be noted that when mixing music and sound effects for film release, engineers always use non-coherent low frequency information in the channel and in the main channels. In other words, the low frequency channel content is different from any other main channel low frequency audio content so that potentially unpredictable acoustic summing in the room is avoided. 20Hz 85Hz 20kHz Mixing Desk Multichannel Monitoring Section O Surround Sound Decoder Outputs S S Subwoofer 120Hz cut-off NO edirection to enter hannel Figure 17 - Multichannel Monitoring with Genelec 6.1 Bass Management. Input band-limited to 120Hz. Genelec Oy, 2002, Version 3 Page 34

6 3.4 Bass Management Bypass Function The channel is unaffected by the bypass function as it affects the main channels only. This means that the F extension of all main channels provided by the subwoofer is bypassed and the full bandwidth is monitored by the main monitors only. The low frequency reproduction of the main channels will extend as low as the main monitor low frequency extension (i.e. for the 1031A: -3dB 47Hz). At the same time, the input remains untouched and the settings concerning routing to the centre channel and bandwidth (85Hz or 120Hz) will be unchanged see Figure 18. The bypass function can be activated in two different ways in the 7060A, 7070A and 7071A subwoofers: 1. Via the mono ¼ jack connector of the front plate using a standard mono shorting switch. 2. Via the J11 remote connector. This optional remote control box provides switching and a status ED for the bypass function. Note that the ¼ jack bypass switch should be set to OFF for the J11 bypass remote control to work and display the status of the bypass function. 20Hz 20kHz Mixing Desk Multichannel Monitoring Section O Surround Sound Decoder Outputs S S Bypassed Subwoofer - edirected to enter - 85Hz fixed - 120Hz fixed Figure 18 - Multichannel Monitoring with Genelec 6.1 Bass Management. All main channels have the subwoofer bypassed, and the channel remains unchanged 3.5 Phase Adjustments When the connection to the Genelec Bass Management is properly done, it is possible and necessary to adjust the Phase at the crossover point once the multichannel system is set-up in the room. Genelec Bass Management allows for 0, 90, 180 or Hz. This guarantees that the main speakers and the subwoofer are in phase at the 85Hz crossover point. An 85Hz tone generator is provided in the Genelec bass management so that it is easy do the necessary on-site phase adjustments. This Phase adjustment has to be done for each subwoofer placed in the room, as they possibly will have different phase adjustments depending on their different physical location in the control room relative to the satellite monitors. This is especially important in the case where one uses a master/slave subwoofer configuration. Genelec Oy, 2002, Version 3 Page 35

7 3.6 Bass oll-off Adjustments Bass oll-off adjustments (-2dB, -4dB and -6dB) can be useful to retrieve a flat subwoofer frequency response. When subwoofers are placed on the floor and against one or two walls, the effect is not only an in-band gain in the frequency response, but often a change in the frequency response shape. The adjustment of the Bass oll-off will enable the user to achieve a flat subwoofer response. 3.7 emote Facilities with J11 onnectors When using the J11 connection, two remote boxes connected to the J11 socket. Both boxes have dual J11 socket for daisy chaining the control signal from one box to the next. 1. Power/Overload ED This remote box has a three-colour ED (green, yellow & red) that copies the status of the three-colour ED mounted on the front plate of the subwoofer. The remote box has two J11 plugs for daisy chaining the control signal to the second remote box. Schematic of the connections is detailed in the operating manual of the subwoofers so that the remote ED can be mounted in any equipment and environment. 2. Bypass Function and +10dB The second remote box has two switches and two status ED s (yellow colour). The ¼ Jack bypass remote has to be OFF for the remote J11 switch to work and display its status. The subwoofer front plate +10dB switch also has to be set to the OFF position (no light) for the remote J11 +10dB switch to work and display its status. 4 Monitoring the with Full ange Monitors Some music recording engineers would rather not use any subwoofers at all as they would rather use full bandwidth monitors without subwoofers. The point is, for example, that in a real orchestra performance all the low frequency information does not come from one physical point in space and that the harmonic content and various reflections in the hall guide our spatial cues towards different places for the instruments in the orchestra. Other engineers in the audio post-production and movie industry sectors want to use subwoofers to reproduce the channel only. In doing so, they emulate the replay conditions in movie theatres. In that situation, the audio content of the channel has to be carefully considered as cancellations may occur between the audio replayed by the main channels and by the subwoofer. Whatever the opinions are, it is also possible to monitor accurately all main channels without a subwoofer when using multiple large full range monitors. The important point is to be able to monitor each channel in its full bandwidth. Therefore, in this case, the five or six full range monitors will reproduce the five or six main channels and the channel should be monitored via one, or more, of the full range monitors. e-routing is required and can be done inside the mixing desk s multichannel monitoring section or in a separate multichannel monitor controller see Figure 19. The most widely used routing for redirecting the channel is to send it to the centre channel. There are also other set-ups where one redirects the channel to front left and front right monitors (note that decoders do this if there is no subwoofer) or even to all three front monitors (this is acoustically the best solution for SP reasons). Genelec Oy, 2002, Version 3 Page 36

8 Note that when routing the same channel to more than one monitor, the total SP of that channel will increase. For example, routing the to the front left and right monitors will add 6dB SP to the overall channel level, therefore, the output level of the channel should be reduced accordingly in the console monitoring matrix output. 20Hz 20kHz Mixing Desk Multichannel Monitoring Section O Surround Sound Decoder Outputs S S outed to:, or /, or // Figure 19 - e-routing the to Full ange Monitors 5 evel Alignment of a Multichannel Surround System 5.1 How to Align the evels of a Multichannel Systems The main goal in the alignment of a multichannel system is to achieve that the subwoofer sound output level (within the band limited subwoofer frequencies) is the same as the sound output level compared to the main loudspeaker system (at other frequencies). The output of the mixing desk or decoder should be connected to the input on the subwoofer. 5.2 What eference evel? To ensure and achieve repeatable results in the finished production, the SMPTE (Society of Motion Pictures and Television) organisation has developed standard monitoring levels for cinema postproduction work. For film mixing, the SMPTE reference is 85dB, with full bandwidth pink noise, read with a sound pressure level meter (SP) set on weighted/slow scale. For release of film material on television, various standards state that the operating mixing level should be somewhat lower so that low-level dialogues, which are easily heard in a quiet and acoustically well treated control room, are mixed slightly higher. This is to ensure that in the home environment with higher background noise levels the dialogue is clearly heard. However, for music mixes, there are no standardised levels as for stereo because the level that the engineer chooses is arbitrary and based on personal taste, as is the level chosen by the end user. Thus, one absolute reference level does not really apply for all multichannel surround sound applications, so the following alignment procedures do not mention specific values but rather the relative levels between the various speakers at the listening position. Genelec Oy, 2002, Version 3 Page 37

9 5.3 alibration of the evel and Frequency esponse with an MS Signal First, there is no point in setting the channel levels until the monitors have been calibrated for their individual frequency responses. Furthermore, there is no point in calibrating the frequency responses using the acoustic tone controls if there are fundamental acoustical problems in the room. evel setting is the last setting to be made once all the other issues have been resolved. The acoustical performance of the main channels and the subwoofer should have a flat frequency response for accurate monitoring. First, make sure that the switch labelled +10dB is set to off position. Then proceed as follows: alibrate the five or six main channel frequency responses using an MS or similar measuring system with the subwoofer bypassed or disconnected. Then connect the GENEE subwoofer as described in the supplied operating manual and adjust the subwoofer level, bass roll-off and phase controls (relative to the centre channel) so that the measured frequency response of the subwoofer AND near field monitor is extended smoothly down to 29Hz (7060A) or 19Hz (7070A and 7071A). Note that there should be no 10dB level changes at low frequency (in the subwoofer bandwidth) compared to the mid and high frequencies as all the headroom level changes of the channel are done electrically in the mixing desk, the bass management system or the decoder. 5.4 Alternative evel alibration Methods If MS type equipment is not available for aligning the system then follow the guidelines that can be found in the operating manual for adjusting the frequency response. emember though that there is still the need to align the system for level. Below are listed two alternative methods but one should note here that the accuracy of these methods depends greatly on the quality and the F response of the SP meter. First, make sure that the switch labelled +10dB is set to off position in the subwoofer bass management. evel alibration using a 1/3 Octave eal Time Analyser, Broadband Pink Noise and an SP Meter onnect the Genelec multichannel system and play broadband pink noise signal (20Hz 20kHz) through the subwoofer and one of the main channels, for example, the centre channel. Set the level of each band on the TA Analyser to read the same value, both in the subwoofer bandwidth AND in the main monitor bandwidth. The specific absolute reference level depends on your application area as mentioned earlier and can be checked using the sound pressure level meter. Place pink noise through the whole system and adjust the level of each channel on the mixing desk to give the same acoustical level in the room. The level depends on the level of the signal but if it is set to -20dBFS rms (-18dBFS rms in Europe) the SP meter should read 85dB for cinema/theatre work, 80-85dB for television and 85-95dB for music. evel alibration using Filtered Pink Noise and an SP Meter You need to have filtered pink noise to calibrate the levels of the subwoofer and the main channels. You can use a copy of the TMH aboratories Multichannel Studio Test Tape that includes the various test signals required. The essential point is that you need two types of test signal: Genelec Oy, 2002, Version 3 Page 38

10 Filtered pink noise from 500Hz to 2kHz for the mid band frequencies of the main monitors. This range is well away from the subwoofer s bandwidth and suffers less from room cancellation effects. Filtered pink noise from 20Hz to 80Hz, to calibrate the subwoofer level. Note: If the recorded level of both the filtered pink noises is 20dBFS rms (-18dBFS rms for Europe) then an absolute level calibration can be made according to your application area, for example, 83dB SP for theatrical work, 78-83dB SP for television and, typically, 83-93dB for music. This level is 2dB lower than for broadband pink noise as there will be less energy in the room due to the limited bandwidth signal. onnect the Genelec multichannel system then play the 500Hz to 2kHz filtered pink noise to adjust each of the main channels individually. Set the SP Meter to -weighting & Slow scale, and note the reading (say it is 83dB SP). All five channels have to be adjusted to this same level. Next, play the 20 to 80Hz filtered pink noise through the subwoofer. The correct adjustment should give a reading 3dB lower than the one for the main channel monitors (in our example, 80dB SP). The reason for the difference in level reading is that most SP meters have a built in HP filter. If there is no HP filter in the SP meter then the reading should be the same as for the main monitors as the bandwidth, and hence energy in the room, of the two signals is the same. Genelec Oy, 2002, Version 3 Page 39