[time-nuts] disciplining sound card

[Azelio Boriani]

The trick here is the 50% divide-by-5, as you can see rather strange but
needed for the XOR.

On Fri, Jul 6, 2012 at 11:14 PM, Azelio Boriani <azelio.boriani at screen.it>wrote:

> OK, here the VHDL of the 24.576MHz synchronizer:
>
>
> LIBRARY IEEE;
> USE IEEE.std_logic_1164.ALL;
> USE IEEE.std_logic_unsigned.ALL;
> ENTITY FRSync IS
> PORT (ClkIN, RefIN: IN std_logic; -- ClkIN is 24.576MHz/1536->16KHz, RefIN
> is 10MHz/625->16KHz
> PWMOut: OUT std_logic);
> END ENTITY FRSync;
> ARCHITECTURE Proc4 OF FRsync IS
> SIGNAL Cnt5A: std_logic_vector (2 downto 0);
> SIGNAL Cnt125 std_logic_vector (6 downto 0);
> SIGNAL Cnt768 std_logic_vector (9 downto 0);
> SIGNAL Cnt2, Cnt5B, Cnt125N, Fr625: std_logic;
> BEGIN
> -- Dividers section
> Div125: PROCESS -- Divide the 10MHz first by 125 then by 5
> BEGIN
> WAIT UNTIL RefIN'EVENT AND RefIN='1';
> IF Cnt125="1111100" THEN
> Cnt125<= (OTHERS=> '0');
> ELSE
> Cnt125<= Cnt125+1;
> END IF;
> END PROCESS Div125;
> Div5A: PROCESS
> BEGIN
> WAIT UNTIL Cnt125(6)'EVENT AND Cnt125(6)='1';
> IF Cnt5A="100" THEN
> Cnt5A<= (OTHERS=> '0');
> ELSE
> Cnt5A<= Cnt5A+1;
> END IF;
> END PROCESS Div5A;
> Div5B: PROCESS
> BEGIN
> WAIT UNTIL Cnt125N'EVENT AND Cnt125N='1';
> Cnt5B<= Cnt5A(1);
> END PROCESS Div5B;
> Div768: PROCESS -- Divide the 24.576MHz first by 768
> BEGIN
> WAIT UNTIL ClkIN'EVENT AND ClkIN='1';
> IF Cnt768="1011111111" THEN
> Cnt768<= (OTHERS=> '0');
> ELSE
> Cnt768<= Cnt768+1;
> END IF;
> END PROCESS Div768;
> Div2: PROCESS -- Then by 2 for a total of 768*2 -> 1536
> BEGIN
> WAIT UNTIL Cnt768(9)'EVENT AND Cnt768(9)='1';
> Cnt2<= NOT Cnt2;
> END PROCESS Div2;
> -- Combinational section
> Cnt125N<= NOT Cnt125(6);
> Fr625<= '1' WHEN Div5A (1)='1' OR Div5B='1' ELSE '0'; -- 50% divide-by-5
> PWMOut<= Fr625 XOR Cnt2; -- The XOR needs exactly a 50% duty cycle
> END ARCHITECTURE Proc4;
>
>
> On Fri, Jul 6, 2012 at 9:40 PM, Tristan Steele <tristan.steele at gmail.com>wrote:
>
>> Another option is to do it yourself, it is one of my projects that is
>> currently in progress. I have some early information at:
>>
>> http://electronics.ozonejunkie.com/category/electronics/time/10mhzaudio/
>>
>> I am aware that the jitter will not be all that low, but I was more
>> interested in longer term stability.  I have since made a smaller, neater
>> board that just needs to go through some testing.  If there is interest, I
>> can post schematics up in the next few days.
>>
>> Tristan
>>
>> On 06/07/2012, at 20:13, lists at lazygranch.com wrote:
>>
>> > I'd suggest hacking USB type soundcards. It is certainly easier to get
>> at the guts.
>> >
>> > There is a Chinese card peddled by a few vendors on ebay that comes in
>> a blue metal case. You can slip out the PCB. The card uses CMedia chips.
>> >
>> > This is the first one I spotted on ebay:
>> >
>> http://item.mobileweb.ebay.com/viewitem?itemId=280506784055&cmd=VIDESC&index=19&nav=SEARCH&nid=33879388392
>> >
>> > I have an older version. These CMedia based cards work well under ALSA.
>> >
>> >
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>
>