Multi-Sub Optimizer Reference (page 3)

Taking Measurements With Your Measurement Software

Before You Begin: Zero Out DSP Delays and Gains, and Clear Out Filters

It's important to realize that the filter parameters and delay and gain values recommended by MSO represent changes relative to the conditions at which the measurements were performed. If MSO recommends a delay of e.g. 7.4 msec for a particular sub, and the measurements were performed with that sub's delay set to 2 msec, the correct final delay would be 9.4 msec, not 7.4 msec. It should be clear that performing the measurements with this sub's delay set to a non-zero value has caused at least two problems. First, the non-zero initial delay requires tedious and error-prone manual calculation of the final delay value. Second, it makes the task of preventing MSO from exceeding the hardware limits of your DSP for e.g. maximum delay more difficult. Assume for this example that a non-HD miniDSP 2x4 device (which has a maximum delay of 7.5 msec) is being used. Setting the maximum allowable delay of the applicable MSO delay block to 7.5 msec would be incorrect for this example, as that would allow for a final delay of up to 9.5 msec. The maximum delay in this case would need to be 5.5 msec, as the delay was 2 msec when the measurement was performed. To avoid such problems, it's much easier to simply zero out all DSP delay and gain/attenuation parameters prior to performing the measurements.

A similar, but even worse situation can occur if the measurements were performed with DSP filters set to a non-flat condition. In the case of miniDSP hardware, exporting a biquad text file from MSO and importing it into a DSP channel that had some filters enabled during the measurements will cause those filters to be erased, not appended to. This virtually guarantees an incorrect result, not just an inconvenience or tendency toward error as with the above delay example.

Imported Measurements Must Be Time-Synchronized

MSO requires that the measurements you import into it be time-synchronized with one another. This means that the relative delays between all loudspeakers measured at a given listening position, including mains and subs, must be preserved. While it might appear that this would not be a problem and would be accomplished automatically by measurement software, this feature is not automatic and special attention is required in order to ensure time synchronization. The method for ensuring that relative delays are preserved involves the use of a timing reference. When using a microphone with an analog output, this is sometimes called a "loopback timing reference". When using a microphone with USB output, the technique used is called an "acoustic timing reference" in the case of Room EQ Wizard. Accomplishing time synchronization of measurements is described in detail in connection with specific software in the sections below.

Caution: Measurements Taken With USB Microphones Require An Acoustic Timing Reference

Time-synchronized measurements with a USB microphone can only be done using Room EQ Wizard version 5.15 or greater, with its "acoustic timing reference" feature. For all other known measurement software, time-synchronized measurements with a USB microphone are not possible. Those familiar with HolmImpulse might be tempted to try HolmImpulse's "time locking" feature to work around this problem, but that workaround fails. The reason is described in the HolmImpulse section below.

Caution: Making Loopback Measurements With HDMI Requires Special Precautions

Special care must be taken when making loopback measurements using HDMI. This is described further in the HDMI measurement section below.

Room EQ Wizard (REW) Considerations

REW is a popular choice for measurement software, and is recommended for use with MSO on all platforms.

To obtain time-synchronized measurements with REW and an XLR microphone such as the Parts Express EMM-6, you must use a separate sound device such as an external USB sound device with a microphone input. In addition, measurements must be performed using a loopback timing reference. To enable the loopback timing reference in REW, go to the Analysis tab of the REW Preferences dialog, and in the section labeled Impulse Response Calculation, make sure the Use Loopback as Timing Reference checkbox is checked before making measurements.

Time-synchronized measurements can be performed with a USB microphone using REW's "acoustic timing reference" feature. To enable it, go to the Analysis tab of the REW Preferences dialog, and in the section labeled Impulse Response Calculation, make sure the Use Acoustic Timing Reference option is chosen before making measurements. This requires REW version 5.15 or later. If you are using REW to measure a two-channel system with a USB microphone, you must use 5.15 beta 6 or later so that the speaker used for the acoustic timing reference can also be measured.

See the REW documentation for measuring with a timing reference for more details, especially the following:

"Note that delay values are not accurate for subwoofer measurements due to the limited bandwidth of the subwoofer response, the delay estimate is based on the location of the peak of the impulse response and subwoofers have a broad peak and a delayed response."

Smoothing in REW

Under certain conditions, REW can apply smoothing to frequency response measurements even if you did not explicitly choose to do so. This can occur when Allow 96 PPO log spacing was enabled in the Analysis tab of the Preferences dialog at the time the measurement was performed. If this is the case, choosing Graph, Remove Smoothing will not remove the smoothing, as the 96 PPO log spacing will remain, and is implicitly associated with 1/48 octave smoothing by REW. To remove the smoothing in a reliable way, a multiple-step process is required.

To avoid having to perform these steps, make sure that the Allow 96 PPO log spacing option in the Analysis tab of the REW Preferences dialog is deselected before performing your measurements.

When exporting unsmoothed data as an FRD file, the frequency resolution of the exported data will be very high. MSO will internally generate its own list of frequencies using a log sequence whose frequency resolution will not exceed 96 points per octave. This is to ensure that all the measurements share a common set of frequencies so they can be added together, taking phase into account, to calculate the combined output of all the subs and main speakers at each frequency.

Using HolmImpulse With The Time-Locking Feature (Windows Only)

Another option for measurements using Windows-based systems is the HolmImpulse freeware. Unfortunately, the HolmImpulse documentation tends to be theoretical in nature and doesn't include much information about how to actually use the software. A worthwhile source of information for learning HolmImpulse is the HolmImpulse tutorial at the Parts Express forum.

HolmImpulse uses a single-input, single-output approach that nonetheless can achieve time-synchronized measurements using the so-called "time locking" feature. The diysubwoofers.org site has a very good step-by-step procedure for using the time-locking feature. A word of caution is in order regarding HolmImpulse and time-locking. When used with sound cards having clocks that are not locked to one another on the record and play sides, the timing reference will shift from one measurement to the next, causing them to be non-time-synchronized and thus unusable with MSO. To find out more about how time-locking can fail in HolmImpulse, see the "Holm Impulse - Timelock, creeping offset?" thread at the Parts Express forum. That thread contains a description of a test that can be run to determine if your sound device is compatible with time-locking. It also contains an explanation by Bill Waslo, author of the Dayton OmniMic measurement software, of the clocking issue as it relates to the potential failure of time-locking in HolmImpulse.

If the clocks on the record-side and playback-side sound devices are not locked to one another, then as Bill Waslo states in the "creeping offset" thread, the time locking cannot work. With that information, it can be seen why using a USB microphone will cause HolmImpulse time-locking to fail. The USB microphone contains an internal A/D converter, so the converter's clock must be contained within the microphone itself. An analog or digital signal from the computer's sound device supplies the input signal to the device under test in this configuration. This signal's clock is internal to the computer itself or in an attached external sound device, so the record and playback clocks are physically different and can't possibly be locked to one another. This will cause time-locking to fail. A similar situation will occur if one attempts to use HolmImpulse time-locking with HDMI supplying the measurement source signal.

It should also be pointed out that these failures of time-locking won't be flagged by the software. In order to find them, you'll need to look for them by performing multiple measurements and checking for consistency.

FuzzMeasure (Mac Only)

FuzzMeasure has a loopback timing reference capability and is therefore recommended for use with MSO, provided a USB microphone is not used. For more information about using loopback with FuzzMeasure, see this Youtube FuzzMeasure tutorial video.

Dayton OmniMic

Dayton OmniMic cannot be used with MSO, as it has no timing reference capability.

General Considerations

When performing subwoofer measurements for use with MSO, it's essential that only one subwoofer be energized at a time for a given subwoofer measurement. Assuming you have a DSP device for performing individual EQ of each sub, this means muting all outputs of the DSP device except the one associated with the sub being measured. If you have more than one sub connected to a given DSP channel, these count as a single "logical sub" for the purposes of MSO, so there is no need to individually energize the physical subs. Multiple subs assigned to a single DSP channel should be physically close to one another.

When performing mains measurements, you have more than one choice. You can choose to integrate center channel plus subs, or left and right plus subs, but not both. Suppose you have a four-channel DSP, a total of four subwoofers, and each DSP channel individually drives a single subwoofer via its associated power amp channel. If you choose to integrate center channel plus subs, each listening position would consist of five measurements as follows.

If you choose to integrate left and right fromt channels plus subs, each listening position would consist of the following five measurements.

If you choose to integrate left and right fromt channels plus subs, and you are unable to energize left and right simultaneously, each listening position would consist of the following six measurements.

The recommended frequency range for the measurements is 10 Hz to 300 Hz. The reason for the high upper limit is to ensure that any filter applied to the mains does not affect the response in the frequency range outside that which is being optimized. This will be explained in more detail in the optimization options section. Measurements should be unsmoothed, or, in the case of newer versions of Room EQ Wizard, variable smoothing can be used. Measurements of mains and subs should be performed over the same frequency range, since MSO determines the frequency range to perform the optimization by finding the highest start frequency and the lowest stop frequency in all imported measurements.

Combined frequency response of mains and subs can be optimized at multiple listening positions, or you can choose to optimize just the subs.

How Many Positions Should Be Measured?

It is recommended that the number of listening positions measured be the same as or a little greater than the number of logical filter channels defined in MSO which contain tunable filters. For example, if you have a miniDSP 8x8 and four subwoofers, each controlled by a separate channel of the miniDSP, and four correspondingly-defined MSO filter channels having tunable filters in each of them, and no EQ of the mains, then you should measure at four or more listening positions. If you wish to EQ the main speakers as well, identical EQ should be used for left and right speakers. Such a shared main speaker EQ counts as an additional logical mains filter channel in MSO. In this case, the number of positions measured should be five or more, as there would be five channels defined in MSO having tunable filters. Your DSP should be set to a flat response with zero delay and zero output attenuation for the measurements. All filters, delays and gains that you add in MSO represent changes from the as-measured condition. Confusion or errors in filter configuration could result later if measurements were performed with some DSP filters or delays already in place. However, it's a good idea to adjust your AVR's sub distance setting to get decent integration of mains and subs before committing to a full set of measurements at multiple listening positions. Be sure to record your AVR's sub distance and sub gain trim settings for the as-measured condition, as you'll need them later. You'll also need to record or remember which DSP channel each sub was connected to when the measurement was performed.

Do Not Try to Simulate Multiple Listening Positions with Measurements Too Closely Spaced

If your system has a single listening seat, MSO is not the appropriate tool to use to optimize sub performance. It's possible to perform measurements in a small "virtual cube" around the single listening position, and MSO will be able to make small improvements in the variation of frequency response with position within that small region. However, the end result can be a significant reduction in maximum system output.

When all the measurement positions are very close together, one or more subs may end up having considerably less flat response than others at all positions in the region. MSO will likely attenuate the response of such subs by a large amount, degrading the maximum system output in the process.

When the measurement region spans a much larger space, a given sub might have a worse response than the others at one measurement position, but be better than the others at a different measurement position. In such a case, more balanced levels will occur for the various subs and MSO will work as intended. If one sub is considerably worse than the others at all measurement positions of a large region, consider relocating the sub if you can.

HDMI Considerations

Do Not Use HDMI Channel 4 (LFE) for Subwoofer Measurements When Using MSO to Integrate Mains and Subs

It might seem that using HDMI channel 4 (the LFE channel) to energize the subs would be the correct way to perform the sub measurements when using a typical AVR or pre-pro. However, if you are using MSO to integrate your main speakers and subs, using the LFE channel to energize the subs is incorrect. The LFE channel has a 10 dB higher gain than all other channels, so the result will be sub measurements that are in error by 10 dB from the correct values. In addition, the low-pass filter assigned to this input, sometimes callet the "LPF of LFE", will often have a different cutoff frequency from the one used in the bass management, and may even have a different slope and alignment as well. For these reasons, channel 4 should never be used to energize the subs for measurements to be used with MSO for integration of subs with main speakers. Instead, energize the subs by using the center-, left-, or right-channel signal, forcing the AVR's bass management to generate the subwoofer signal.

It's Okay to Use HDMI Channel 4 (LFE) for Subwoofer Measurements When Using MSO to Optimize Subs Only

For sub-only optimization, using HDMI channel 4 is okay, as the 10 dB level increase of the LFE channel relative to the main speakers is not a problem when you're measuring only subs. When optimizing only subs, it's best to set the LPF of LFE to the highest allowable setting to get the flattest possible sub response up to the highest measurement frequency.

Using HDMI With the REW Acoustic Timing Reference and USB Microphone

If you're a Windows user, it's a good idea to read AustinJerry's guide (in PDF form), called "Getting Started With REW: A Step-by-Step Guide" before doing measurements with HDMI and a USB microphone (which requires an acoustic timing reference). This document has many details and step-by-step instructions for how to get everything set up and running. Even experienced users will likely find something useful that they may not have known before.

The table below shows the HDMI channel numbers to use with REW to energize the corresponding channels in a 7.1 setup. When using a Windows computer with an HDMI interface with REW, use the ASIO4All software to enable the measurement of all available channels if the hardware supports it.

Channel Number Channel Name
1 Left
2 Right
3 Center
4 LFE
5 Rear left
6 Rear right
7 Side left
8 Side right

Channel number vs. physical channel for 7.1 audio

When using the REW acoustic timing reference, it's essential that the speaker you use as the timing reference be a full-range type. You cannot use a subwoofer as the acoustic timing refference. The example below demonstrates the use of the acoustic timing reference when measuring a system having a Left channel, Right channel, and four subs. It's also essential that you use the same channel as the acoustic timing reference for all measurements. The example below uses the right channel for this purpose. This example assumes the usage of a conventional AVR or pre-pro, together with a DSP device that supports 4 subs connected to the AVR's sub out. Notice that for the subwoofer measurements, the REW output is set to the left channel, but the left speaker is muted or disconnected, so that no sound comes from it. In this case, the left channel energizes the subs through the AVR's bass management. These measurements need to be repeated at each listening position.

Left Channel, Right Channel and Subs

For the arrangement above, each measurement position becomes an MSO measurement group with the above six measurements. MSO performs a complex summation of these six measurements to obtain the data both for this measurement group's graphs and for its optimization. MSO will sense that there are two main measurements in the measurement group, and will boost all subwoofer data by 6 dB before summation to take into account the fact that both main speakers are summed together, but the subs were only energized by a single main channel (not both main channels simultaneously).

The following example is for a center channel and four subs, using the right channel as the acoustic timing reference. These measurements need to be repeated at each listening position.

Center Channel and Subs

Hardware Loopback Measurements Using HDMI

In order to do hardware loopback measurements with HDMI, you'll need to use either a pre-pro or an AVR with preamp outputs. You'll need to take the loopback signal from the preamp output corresponding to the HDMI input used for the timing reference signal. The setup below is one example of this technique.