Phred33 Why subtract 33

Question

[eledu998]

Hi everyone!

I 've been searching for long time why subtract 33 in :

ord(<ASCII>)-33.

but I cannot find the answer. Somebody could please share a link, give some information where to find the answer or teach me why?

Many thanks in advance!!

[eledu998]

Many thanks for your answer , yeah I already knew that.

there is no characterer below 64 in fastq files right? or at least in the ones I handle

ord("@") is 64, so what is the need for subtract 33??

Many thanks in advance for your time and kind regards

[Pierre Lindenbaum]

there is no characterer below 64 in fastq files right?

it depends of the fastq encoding. https://en.wikipedia.org/wiki/FASTQ_format#Encoding

Solexa/Illumina 1.0 format can encode a Solexa/Illumina quality score from -5 to 62 using ASCII 59 to 126

Answer 1

[Pierre Lindenbaum]

this is the ASCII code: everything below 33 (32=(SPACE)) is hidden/special. So, to get a printable character on the screen you need to start a ASCII-Code==33.

   Oct   Dec   Hex   Char                        Oct   Dec   Hex   Char
   ────────────────────────────────────────────────────────────────────────
   000   0     00    NUL '\0'                    100   64    40    @
   001   1     01    SOH (start of heading)      101   65    41    A
   002   2     02    STX (start of text)         102   66    42    B
   003   3     03    ETX (end of text)           103   67    43    C
   004   4     04    EOT (end of transmission)   104   68    44    D
   005   5     05    ENQ (enquiry)               105   69    45    E
   006   6     06    ACK (acknowledge)           106   70    46    F
   007   7     07    BEL '\a' (bell)             107   71    47    G
   010   8     08    BS  '\b' (backspace)        110   72    48    H
   011   9     09    HT  '\t' (horizontal tab)   111   73    49    I
   012   10    0A    LF  '\n' (new line)         112   74    4A    J
   013   11    0B    VT  '\v' (vertical tab)     113   75    4B    K
   014   12    0C    FF  '\f' (form feed)        114   76    4C    L
   015   13    0D    CR  '\r' (carriage ret)     115   77    4D    M
   016   14    0E    SO  (shift out)             116   78    4E    N
   017   15    0F    SI  (shift in)              117   79    4F    O
   020   16    10    DLE (data link escape)      120   80    50    P
   021   17    11    DC1 (device control 1)      121   81    51    Q
   022   18    12    DC2 (device control 2)      122   82    52    R
   023   19    13    DC3 (device control 3)      123   83    53    S
   024   20    14    DC4 (device control 4)      124   84    54    T
   025   21    15    NAK (negative ack.)         125   85    55    U
   026   22    16    SYN (synchronous idle)      126   86    56    V
   027   23    17    ETB (end of trans. blk)     127   87    57    W
   030   24    18    CAN (cancel)                130   88    58    X
   031   25    19    EM  (end of medium)         131   89    59    Y
   032   26    1A    SUB (substitute)            132   90    5A    Z
   033   27    1B    ESC (escape)                133   91    5B    [
   034   28    1C    FS  (file separator)        134   92    5C    \  '\\'
   035   29    1D    GS  (group separator)       135   93    5D    ]
   036   30    1E    RS  (record separator)      136   94    5E    ^
   037   31    1F    US  (unit separator)        137   95    5F    _
   040   32    20    SPACE                       140   96    60    `
   041   33    21    !                           141   97    61    a
   042   34    22    "                           142   98    62    b