For those who have followed my work with the Pod HD, you know that I have often used "dual cabs" in my patches.  For those who don't, see here to catch up: http://foobazaar.com/podhd/toneGuide/cabsMics#dualCab - basically I'm using the same amp in a dual amp patch but varying the cab/mic used in each channel.  I combine a "bright cab" with a "dark cab" to get a consistent, high-quality frequency response from the uber-lows to ultra-highs.

A big part of dialing in these tones revolves around one cab/mic being out-of-phase with the other.  Previously, I used two crude methods to work-around this issue: either find combinations that seemed to be in-phase, or try adding a neutral EQ or two to one Channel only to see if it improved the tone.  Today, I present you a solid set of research data that more accurately demonstrates and rectifies the issue.

But first, here is a small sample of my research applied to a HD 500 setlist.  Notice the tone is consistent and rich throughout the entire frequency spectrum.  I cannot get close to this quality of tone with a single amp/cab.  If you can, I'd like to see your patch.

http://foobazaar.com/podhd/toneGuide/dualCabs.h5s

This is just the tip of the iceberg.  I plan to complete this setlist with other cabs serving as the "bright cab" other than the Tread V-30 and 57 on axis.  I also want to make multiple setlists for each of the high gain amps.  I also want to add the Marshall T75 cab.  I want everyone who enjoys high gain to hear what the Pod HD is truly capable of.

The naming convention for each patch is "2_X_Ym_Zn" where 2 means dual cab setlist, X means amp used, Y means cabinet in Channel A, m means mic in Channel A, Z means cabinet in Channel B, and n means mic in Channel B.  For instance, the first patch is "2_R_R-_R/" - this means dual cab setlist, Rectifier amp, Rectifier cab with 57 on axis mic in Channel A, Rectifier cab with 57 off axis mic in Channel B.  Below lists all possibilities for the legend:

Amps

  J - JCM-800

  R - Rectifier

  U - Uberschall

  F - Fireball

Cabs

  R - Tread V-30

  H - Hiway

  X - XXL

  G - Greenbacks

  U - Uber

Mics

  - - 57 on axis

  / - 57 off axis

  9 - 409 Dyn

  2 - 421 Dyn

  4 - 4038 Ribbon

  1 - 121 Ribbon

  6 - 67 Cond

  8 - 87 Cond

There are 3 links below.  The first is a spreadsheet with a matrix of every possible combination of cab/mic with another, for the Tread V-30, Hiway, XXL, Greenbacks, and Uber 4x12 cabs with all available mics with the delay required to make them in-phase represented in a number of samples assuming a 96 kHZ sample rate.  The second is a list of relatively tone-transparent effects, the settings I used for my research, and the delay time in samples the effect adds to the signal.  The third is a list of a combination of effects, useful to get liquidity from the second link.

http://foobazaar.com/podhd/toneGuide/CabsDelayTimes.xlsx

http://foobazaar.com/podhd/toneGuide/FXDelayTimes.xlsx

http://foobazaar.com/podhd/toneGuide/FXCombinationDelayTimes.xlsx

EDIT: The links below replace those above.  See my last post in this thread for more details on changes.

Online Google Sheets version:

https://docs.google.com/spreadsheet/ccc?key=0AsSFadRSa9zYdG1kc1J2b1ZvRFVlUVIydXRMVVoyZnc&usp=sharing

Downloadable Excel version:

http://foobazaar.com/podhd/toneGuide/CabsDelayTimes_V3.xlsx

So here's how to apply this knowledge.  First look up any pair of cab/mic you want to use with any other cab/mic in the CabDelayTimes spreadsheet.  Channel A's cab/mic is listed on the x axis, Channel B on the Y axis.  Where these rows and columns meet, you'll see a number.  That tells you the number of samples you need to delay Channel B to get phase correction.  If the number is negative, that means you need to apply the delay to Channel A (if so, it might be simpler to simply switch the cab/mic in Channel B to Channel A and vice versa).

Once you have found the delay number, you need to add effects to channels A and B to get the specified delay.  You'll notice in the FXDelayTimes spreadsheet, the smallest delay time for any effect is 6 samples.  To get values less than that you need to add a larger delay to one Channel and a shorter delay to the other Channel.  For instance, to get a 4 sample delay in Channel B, you'd put a Parameter EQ in Channel A and a Blue Comp in Channel B.  This puts 10 samples in Channel B and 6 sample in Channel A, giving you a difference of 4 samples in Channel B.  This is where the FXCombinationDelayTimes spreadsheet is useful - you can quickly look up combinations that will yield delay samples in increments of 1 sample from 1 to 20+.

A few notes:

Even with phase correction, the bass can get a bit woofy sounding if you dial in bass on both cabinets.  I set the Low Cut Cab DEP to around 50% (260 HZ) on my "bright cab" to prevent having both too much bass and it sounding a bit "off".  The bass is going to sound cleaner and tighter from the "dark cab" anyway.

On my "dark cab", I find even with phase correction, the mixed high ends sound kinda fake together, especially when using different mics.  So I generally set the amp's Treble control to 0% on my "dark cab".

Most of the compressors additionally include a LP filter.  This is quite evident when they are applied to whichever channel you use as your "bright cab".  Thus, I try to avoid using them to delay the signal on my "bright cab".  If I need to use one to get the specified phase correction in my document, I just ignore that and get as close as I can using an EQ or something that definitely won't kill my high end.  For instance, if I need an 8 sample delay, the closest is the Vetta Juice at 7.5 samples, but I instead use a Mid-Focus EQ with 6.5 samples.  On my "dark" cab, this is not an issue.  I'm ok with dialing out the high-end there, and will use whatever gets me closest to the research.

Many cab/mic combinations are currently impractical.  Using 3-4 effects to achieve phase correction is a huge drain on DSP and effect blocks.  I hope my research isn't the end result, but a starting point for individuals to realize how powerful the onboard cab/mic sims can be if they could be dialed in as mentioned.  The real end-game is for Line 6 to implement a feature to be able to delay each channel on the Mixer block by samples in increments of 1 from 0-60.  This would require a buffer maximum of 60 samples, which at 24 bits is only 180 bytes of memory.  It should require little to no DSP, similar to the Volume effect.  Then any dual cab combination would become practical, even when using some DSP-expensive effects.

Related threads:

Feature Request

http://line6.com/support/message/390465

TheGearPage Thread about Pod HD timing

http://www.thegearpage.net/board/showthread.php?t=1138483

TheGearPage thread discussing the same effect delay phenomenon

http://www.thegearpage.net/board/showthread.php?t=964530

Edit: Some have mentioned possibly making this adjustment automatic in a feature request.  I agree.  Some effects even change their delay value, making it impossible to be manually synced.

the pitch glide is the craziest example.  when you turn it on it's at a certain number of samples, but as you use it, the exact delay changes.  so if it's at unison and there's 20 sample delay, then you move it up an octave and back to unison, now the delay might be like 30 samples.

the mid-focus eq also seems to have slight variation of its delay as you move the LP freq and Q.

i think the best case fix would be a system setting that can be set either to auto or manual or mixed.  auto would behave as you describe - the two channels are automatically synced.  manual is straight sample-based delays adjustible on either channel as i originally described.  mixed would automatically update the values, but you could set it to be always at a certain amount out of sync.

The way auto would work is it would sum the total delay in samples for each channel in real time depending on the effect, amp, and cab/mic choices.  Whichever channel had the least latency would have latency added to match the other channel.  This would occur in real-time, so that toggling effects on/off, or changing parameters would not cause the channels to become out of sync.

i think all modes would be important because when mixing certain cab/mics, some people may find the comb filter effect to be pleasing, reducing some harsh high-end.  So manual/mixed would be as valuable as auto to these people.  and some people may even want to toggle effects on/off to engage/disengage the comb filter, so they'd prefer manual over mixed.