[hpsdr] Strange Janus effect

[Phil Harman]

Hi Dave,

If you have a Penelope board then take your receive audio from this. It  has 
a full 16 bit DAC which will resolve this issue.

73's Phil....VK6APH


----- Original Message ----- 
From: "David Woodhead" <dwoodhead at bms-inc.com>
To: <hpsdr at lists.hpsdr.org>
Sent: Saturday, May 24, 2008 12:09 AM
Subject: Re: [hpsdr] Strange Janus effect


> ***** High Performance Software Defined Radio Discussion List *****
>
> I was wondering if a fix was ever implemented for this issue; Janus
> sound distorted when RF gain control is reduced ?
>
> I remember a few posts on the subject and one in particular from Phil
> below. I have found the audio quality of the the D44 to be noticably
> cleaner than Janus when the RF gain control is reduced.
>
> Thanks,
>
> David
> KM5TZ
>
>
> -----Original Message-----
> From: hpsdr-bounces at lists.hpsdr.org
> [mailto:hpsdr-bounces at lists.hpsdr.org] On Behalf Of Phil Harman
> Sent: Thursday, October 11, 2007 6:21 PM
> To: a.groff
> Cc: hpsdr at lists.hpsdr.org; flexradio at flex-radio.biz
> Subject: Re: [hpsdr] Strange Janus effect
>
> ***** High Performance Software Defined Radio Discussion List *****
>
>
> Hi Al,
>
> This is what I suspect you are seeing.
>
> There are two audio outputs from Janus, one from the TLV320 and the
> other from the PWM DAC.  We originally intended the TLV320 to drive
> headphones/speakers/line out and the PWM DAC to provide I and Q to the
> SDR Tx.
>
> Very late in the design I spoke to Bob N4HY who let me know that he was
> considering altering PowerSDR to move the I and Q signals away from
> baseband to get rid of the low frequency noise that some sound cards
> produce.
>
> This change would have meant that we could not have used the I and Q
> outputs to drive the Tx since they were intended to work at baseband.
> So we modified the FPGA code to send the Rx audio to the Janus I&Q
> outputs and the I&Q signals to the TLV320.
>
> The PWM DAC that is now used for Rx audio uses about 11 bits as apposed
> to the 16 bits that the TLV320 uses.  So,if you reduce AGC-T (RF Gain)
> and increase the AF gain you reach a point where you starve the PWM DAC
> of bits and it starts to distort.  I can hear this happening if I set
> the controls as above.
>
> There are a number of options (since we use FPGAs!)
>
> 1. Keep the RF gain at the level at which the band noise just does not
> activate the AGC - which is how it was intended to be used.
>
> 2. Change the Ozy FPGA code so that we swap the audio outputs - we will
> need you to run some test on the I&Q outputs to check the S/N, phase and
> amplitude balance etc is OK. This should be OK unless PowerSDR changes
> to non baseband I&Q signals some time in the future.
>
> 3. Wait for Penelope and/or Mercury since both have additional TLV320s
> which we can use for Rx audio!
>
> Sorry for the long winded answer and hope this makes sense.
>
> 73's Phil....VK6APH
>
>
>
> Quoting "a.groff" <k0vm at mchsi.com>:
>
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> On Aug 30 I posted to the HPSDR mailing list
>> ----------------------------------------------------------------------
>> ---
>>
>> Subject: [hpsdr] Strange Janus effect...
>>
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> I am noticing a strange cross-talk ( ?? ) effect with Janus (& OZY ? )
>
>> when used with SDR-1000 & PowerSDR  (v1.9.1 svn 1354, v1.10.2 ) that I
>
>> do not notice with the D44 sound card...
>>
>> I notice this effect in the receive audio when I start to decrease the
>
>> AGC-T ( RF Gain ) control in Power SDR.  As the received signal starts
>
>> to disappear from audibility, it is replaced by very distorted replica
>
>> at a reduced amplitude.  The effect was first observed in SSB modes
>> but is also present it AM mode ( maybe more so. )
>>
>> Does you one else have this effect or do I have a problem with my
>> Janus/Ozy/system ??  How would I start to isolate the problem?
>>
>> AL, K0VM
>
>
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