In article <1187913274.631616.290380@m37g2000prh.googlegroups.com>, 
Andrew.J.Hunter74@gmail.com says...
>
>
>Dear Group,
>
>I have a 16th order Chebyshev type 2 low pass IIR filter and I'd like
>add a couple of second order allpass stages to linearize the phase
>response. Ideally, the resulting filter should have an equiripple
>reponse around an ideal linear response.
>
>I've googled quite extensively, but I can't find any information on
>how to design the allpass sections.
>Does anybody know of a techique?

s/FILSYN (http://www.alkeng.com/sfilsyn.html) will do this. However, you 
may find that you need significantly more than two 2nd order allpass 
sections to get good group delay EQ, particularly if you want to 
equalize all the way to the edge of the passband. You can get a good 
feel for the magnitude of the problem by plotting the group delay vs. 
frequency of the original Cheby. If the group delay spikes over a narrow 
range around the cutoff frequency and its maximum value is much greater 
than its value at 0 Hz, then the required group delay equalizer is going 
to be high-order because it has to add a lot of group delay to all 
frequencies below the "spike" and it is going to have to be selective 
enough to incorporate the "spike" smoothly in the final equalized group 
delay curve.

On 24 Aug., 01:54, Andrew.J.Hunte...@gmail.com wrote:
> Dear Group,
>
> I have a 16th order Chebyshev type 2 low pass IIR filter and I'd like
> add a couple of second order allpass stages to linearize the phase
> response. Ideally, the resulting filter should have an equiripple
> reponse around an ideal linear response.
>
> I've googled quite extensively, but I can't find any information on
> how to design the allpass sections.

Google -> iir filter phase equalization -> I'm feeling lucky!

On Aug 23, 7:54 pm, Andrew.J.Hunte...@gmail.com wrote:
> Dear Group,
>
> I have a 16th order Chebyshev type 2 low pass IIR filter and I'd like
> add a couple of second order allpass stages to linearize the phase
> response. Ideally, the resulting filter should have an equiripple
> reponse around an ideal linear response.
>
> I've googled quite extensively, but I can't find any information on
> how to design the allpass sections.
> Does anybody know of a techique?
>
> Thanks,
> Andrew

If you have the whole sequence in a file or memory, you can use
something like Matlab's 'filtfilt' routine. If you don't you can
approximate with a FIR and implement using fast convolution
techniques.

John

On Aug 24, 11:54 am, Andrew.J.Hunte...@gmail.com wrote:
> Dear Group,
>
> I have a 16th order Chebyshev type 2 low pass IIR filter and I'd like
> add a couple of second order allpass stages to linearize the phase
> response. Ideally, the resulting filter should have an equiripple
> reponse around an ideal linear response.
>
> I've googled quite extensively, but I can't find any information on
> how to design the allpass sections.
> Does anybody know of a techique?
>
> Thanks,
> Andrew

If you buy Filter Solutions the software does it for you and gives you
teh circuit diagram as well!

Dear Group,

I have a 16th order Chebyshev type 2 low pass IIR filter and I'd like
add a couple of second order allpass stages to linearize the phase
response. Ideally, the resulting filter should have an equiripple
reponse around an ideal linear response.

I've googled quite extensively, but I can't find any information on
how to design the allpass sections.
Does anybody know of a techique?

Thanks,
Andrew