Re: [RFC] TLV entry for linear volume

[James Courtier-Dutton <James@superbug.co.uk>]

Takashi Iwai wrote:
> At Thu, 24 Aug 2006 11:16:45 +0200,
> I wrote:
>   
>> The difference between us is just how to express and pass the
>> coefficeints.  My suggestion is to pass the values given in the
>> datasheet as they are, min and max dB.
>>     
>
> The below is the latest version I have on my local tree.
> It's against HG tree.
>
>
> Takashi
>   
+#ifndef HAVE_SOFT_FLOAT
+    case SND_CTL_TLVT_DB_LINEAR: {
+        int mindb = rec->db_info[2];
+        int maxdb = rec->db_info[3];
Maybe I am unclear about what rec->min is, but I though that it was the 
lowest value the volume(old % scale) could be.
So, what exactly is this section trying to do? Just handling error cases?
+        if (volume <= rec->min || rec->max <= rec->min)
+            *db_gain = mindb;
+        else if (volume >= rec->max)
+            *db_gain = maxdb;
+        else {

This makes sense. (part A), when combined with (part B)
+            double val = (double)(volume - rec->min) /
+                (double)(rec->max - rec->min);

This special case is for when mindb = negative infinity.
+            if (mindb <= SND_CTL_TLV_DB_GAIN_MUTE)
+                *db_gain = (long)(100.0 * 20.0 * log10(val)) +
+                    maxdb;
+            else {

This makes sense. (Part B) when combined with (Part A)
It covers a good general case of mapping (linear_min...linear_max) onto 
(dB_min...dB_max)
+                /* FIXME: precalculate and cache these values */
+                double lmin = pow(10.0, mindb/20.0);
+                double lmax = pow(10.0, maxdb/20.0);
+                val = (lmax - lmin) * val + lmin;
+                *db_gain = (long)(100.0 * 20.0 * log10(val));
+            }
+        }
+        return 0;
+    }
+#endif


Summary, I am happy with this now.

James


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